Determinants of SARS-CoV-2 anti-spike antibody levels following BNT162b2 vaccination: cross-sectional analysis of 6,000 SIREN study participants (preprint)

Background: Understanding immunological responses to SARS-CoV-2 vaccinations is integral to the management of SARS-CoV-2. We aimed to investigate determinants of antibody response to the BNT162b2 vaccine. Methods: A cross-sectional analysis of anti-spike binding antibodies in serum samples from healthcare workers after one or two doses. Post-vaccination interval was restricted to [≥]21 days after dose 1, [≥]14 days after dose 2. The primary outcome was anti-S titres with explanatory variables dose, previous infection, dosing interval, age, ethnicity, and comorbidities. Multivariable linear regression was also conducted. Results: Participants (n=5,871) included 3,989 post-dose 1, 1,882 post-dose 2. In SARS-CoV-2 infection naive, 99.65% seroconverted after dose 1 and >99.9% seroconverted after dose 2. Geometric mean anti-S titre in the naive cohort was 75.48 Binding Antibody Units/ml after dose 1, 7,049 BAU/ml after dose 2. Anti-S titres were higher in those with previous infection (2,111 BAU/ml post-dose 1, 16,052 BAU/ml post-dose 2), and increased with time between infection and vaccination: 3 months 1,970 (1,506-2,579) vs 9 months; 13,759 (8,097-23,379). Longer dosing intervals increased antibody response post-dose 2: 11-fold higher with a longer interval (>10 weeks) than those with shorter intervals, across all age-groups. Younger participants had higher mean titres (>2.2-fold higher). Multivariable regression modelling corroborated the above associations, and also found higher titres associated with being female or from an ethnic minority but lower titres among immunocompromised participants. Conclusion: The number of antigen exposures and timing between vaccinations plays a significant role in the magnitude of the post-vaccination antibody response, with implications for long-term protection and post-booster antibody responses.

Emergence of the Delta Variant and risk of SARS-CoV-2 infection in secondary school students and staff: prospective surveillance in 18 schools, England (preprint)

Background The role of educational settings on SARS-CoV-2 infection and transmission remains controversial. We investigated SARS-CoV-2 infection, seroprevalence and seroconversions rates in secondary schools during the 2020/21 academic year, which included the emergence of the more transmissible Alpha and Delta variants, in England. Methods The UK Health Security Agency (UKHSA) initiated prospective surveillance in 18 urban English secondary schools. Participants had nasal swabs for SARS-CoV-2 RT-PCR and blood sampling for SARS-CoV-2 Nucleoprotein and Spike protein antibodies at the start (Round 1: September-October 2020) and end (Round 2: December 2021) of the autumn term, when schools reopened after national lockdown was imposed in January 2021 (Round 3: March-April) and end of the academic year (Round 4: May-July). Findings We enrolled 2,314 participants (1277 students, 1037 staff). In-school testing identified 31 PCR-positive participants (20 students, 11 staff). Another 247 confirmed cases (112 students, 135 staff) were identified after linkage with national surveillance data, giving an overall positivity rate of 12.0% (278/2313; staff [14.1%, 146/1037] vs students [10.3%, 132/1276; p=0.006). Nucleoprotein-antibody seroprevalence increased for students and staff between Rounds 1-3 but changed little in Round 4, when the Delta variant was the dominant circulating strain. Overall, Nucleoprotein-antibody seroconversion was 18.4% (137/744) in staff and 18.8% (146/778) in students, while Spike-antibody seroconversion was higher in staff (72.8% (525/721) than students (21.3%, 163/764) because of vaccination. Interpretation SARS-CoV-2 infection and transmission in secondary schools remained low when community infection rates were low because of national lockdown, even after the emergence of the Delta variant

Effectiveness and durability of protection against future SARS-CoV-2 infection conferred by COVID-19 vaccination and previous infection; findings from the UK SIREN prospective cohort study of healthcare workers March 2020 to September 2021 (preprint)

BackgroundUnderstanding the duration and effectiveness of infection and vaccine-acquired SARS-CoV-2 immunity is essential to inform pandemic policy interventions, including the timing of vaccine-boosters. We investigated this in our large prospective cohort of UK healthcare workers undergoing routine asymptomatic PCR testing. MethodsWe assessed vaccine effectiveness (VE) (up to 10-months after first dose) and infection-acquired immunity by comparing time to PCR-confirmed infection in vaccinated and unvaccinated individuals using a Cox regression-model, adjusted by prior SARS-CoV-2 infection status, vaccine-manufacturer/dosing-interval, demographics and workplace exposures. ResultsOf 35,768 participants, 27% (n=9,488) had a prior SARS-CoV-2 infection. Vaccine coverage was high: 97% had two-doses (79% BNT162b2 long-interval, 8% BNT162b2 short-interval, 8% ChAdOx1). There were 2,747 primary infections and 210 reinfections between 07/12/2020 and 21/09/2021. Adjusted VE (aVE) decreased from 81% (95% CI 68%-89%) 14-73 days after dose-2 to 46% (95% CI 22%-63%) >6-months; with no significant difference for short-interval BNT162b2 but significantly lower aVE (50% (95% CI 18%-70%) 14-73 days after dose-2 from ChAdOx1. Protection from infection-acquired immunity showed evidence of waning in unvaccinated follow-up but remained consistently over 90% in those who received two doses of vaccine, even in those infected over 15-months ago. ConclusionTwo doses of BNT162b2 vaccination induce high short-term protection to SARS-CoV-2 infection, which wanes significantly after six months. Infection-acquired immunity boosted with vaccination remains high over a year after infection. Boosters will be essential to maintain protection in vaccinees who have not had primary infection to reduce infection and transmission in this population. Trial registration numberISRCTN11041050

Serological responses to COVID-19 booster vaccine in England (preprint)

Importance: There are limited data on immune responses after COVID-19 vaccine boosters in individuals receiving primary immunisation with BNT162b2 (Pfizer-BioNTech) or AZD1222 (AstraZeneca) Objective: To assess SARS-CoV-2 antibody responses before and after booster vaccination with BNT162b2 in adults receiving two BNT162b2 or AZD1222 vaccine doses at least 6 months previously, as part of the United Kingdom national immunisation schedule Design: Prospective, cohort study Setting: London, England Participants: 750 immunocompetent adults aged [≥]50 years Interventions: A single dose of BNT162b2 administered at least six months after primary immunisation with two doses of BNT162b2 given <30 days apart (BNT162b2-control) or [≥]30 days apart (BNT162b2-extended) compared to AZD1222 given [≥]30 days apart (AZD1222-extended) Main Outcome and Measures: SARS-CoV-2 spike protein antibody geometric mean titres (GMTs) before and 2-4 weeks after booster Results: Of 750 participants, 626 provided serum samples for up to 38 weeks after their second vaccine dose. Antibody GMTs peaked at 2-4 weeks after the second dose, before declining by 68% at 36-38 weeks after dose 2 for BNT162b2-control participants, 85% at 24-29 weeks for BNT162b2-extended participants and 78% at 24-29 weeks for AZD1222-extended participants. Antibody GMTs was highest in BNT162b2-extended participants (942 [95%CI, 797-1113]) than AZD1222-extended (183 [124-268]) participants at 24-29 weeks or BNT162b2-control participants at 36-38 weeks (208; 95%CI, 150-289). At 2-4 weeks after booster, GMTs were significantly higher than after primary vaccination in all three groups: 18,104 (95%CI, 13,911-23,560; n=47) in BNT162b2-control (76.3-fold), 13,980 (11,902-16,421; n=118) in BNT162b2-extended (15.9-fold) and 10,799 (8,510-13,704; n=43) in AZD1222-extended (57.2-fold) participants. BNT162b2-control participants (median:262 days) had a longer interval between primary and booster doses than BNT162b2-extended or AZD1222-extended (both median:186 days) participants. Conclusions and Relevance: We observed rapid serological responses to boosting with BNT162b2, irrespective of vaccine type or schedule used for primary immunisation, with higher post-booster responses with longer interval between primary immunisation and boosting. Boosters will not only provide additional protection for those at highest risk of severe COVID-19 but also prevent infection and, therefore, interrupt transmission, thereby reducing infections rates in the population. Ongoing surveillance will be important for monitoring the duration of protection after the booster.

mRNA COVID-19 vaccines induce enhanced antibody and cellular responses compared to ChAdOx1 or natural infection in children (preprint)

We present a comprehensive analysis of antibody and cellular responses in children aged 12-16 years who received COVID-19 vaccination with ChAdOx1 (n=6) or mRNA vaccine (mRNA-1273 or BNT162b2, n=9) using a 12-week extended-interval schedule. mRNA vaccination of seropositive children induces high antibody levels, with one dose, but a second dose is required in infection-naïve children. Following a second ChAdOx1 dose, antibody titres were higher than natural infection, but lower than mRNA vaccination. Vaccination induced live virus neutralising antibodies against Alpha, Beta and Delta variants, however, a second dose is required in infection-naïve children. We found higher T-cell responses following mRNA vaccination than ChAdOx1. Phenotyping of responses showed predominantly early effector-memory CD4 T cell populations, with a type-1 cytotoxic cytokine signature, with IL-10. These data demonstrate mRNA vaccination induces a co-ordinated superior antibody and robust cellular responses in children. Seronegative children require a prime-boost regime for optimal protection.

Implementation and extended evaluation of the Euroimmun Anti-SARS-CoV-2 IgG assay and its contribution to the United Kingdom's COVID-19 public health response (preprint)

In March 2020, the Rare and Imported Pathogens Laboratory at Public Health England, Porton Down, was tasked by the Department of Health and Social Care with setting up a national surveillance laboratory facility to study SARS-CoV-2 antibody responses and population-level sero-surveillance in response to the growing SARS-CoV-2 outbreak. In the following 12 months, the laboratory tested more than 160,000 samples, facilitating a wide range of research and informing PHE, DHSC and UK government policy. Here we describe the implementation and use of the Euroimmun anti-SARS-CoV-2 IgG assay and provide an extended evaluation of its performance. We present a markedly improved sensitivity of 91.39% ([≥]14 days 92.74%, [≥]21 days 93.59%) compared to our small-scale early study, and a specificity of 98.56%. In addition, we detail extended characteristics of the Euroimmun assay: intra- and inter-assay precision, correlation to neutralisation and assay linearity.

Nucleocapsid antibody positivity as a marker of past SARS-CoV-2 infection in population serosurveillance studies: impact of variant, vaccination, and choice of assay cut-off (preprint)

Serological surveillance studies sometimes use presence of anti-nucleocapsid antibody as a marker of natural SARS-CoV-2 infection. We explore seroconversion rates and antibody titres following Alpha and Delta variant infections, and vaccine breakthrough infections. We find lower seroconversion rates particularly following Alpha-variant vaccine breakthrough infections. We re-evaluate assay performance with a mix of past waned infections and recent breakthrough infections, that is relevant to current serological surveillance.

Higher serological responses and increased vaccine effectiveness demonstrate the value of extended vaccine schedules in combatting COVID-19 in England (preprint)

IntroductionIn January 2021, the UK decided to prioritise the delivery of the first dose of BNT162b2 (Pfizer/BioNTech) and AZD1222 (AstraZeneca) vaccines by extending the interval until the second dose up to 12 weeks. MethodsSerological responses were compared after BNT162b2 and AZD1222 vaccination with varying intervals in uninfected and previously-infected adults aged 50-89 years. These findings are evaluated against real-world national vaccine effectiveness (VE) estimates against COVID-19 in England. ResultsWe recruited 750 participants aged 50-89 years, including 126 (16.8%) with evidence of previous infection; 421 received BNT162b2 and 329 and AZD1222. For both vaccines, over 95% had seroconverted 35-55 days after dose one, and 100% seroconverted 7+ days after dose 2. Following a 65-84 day interval between two doses, geometric mean titres (GMTs) at 14-34 days were 6-fold higher for BNT162b2 (6703; 95%CI, 5887-7633) than AZD1222 (1093; 806-1483), which in turn were higher than those receiving BNT162b2 19-29 days apart (694; 540 - 893). For both vaccines, VE was higher across all age-groups from 14 days after dose two compared to one dose, but the magnitude varied with interval between doses. Higher two-dose VE was observed with >6 week intervals between BNT162b2 doses compared to the authorised 3-week schedule, including [≥]80 year-olds. ConclusionOur findings support the UK approach of prioritising the first dose of COVID-19 vaccines, with evidence of higher protection following extended schedules. Given global vaccine constraints, these results are relevant to policymakers, especially with highly transmissible variants and rising incidence in many countries. FundingPublic Health England

Emergence of SARS-CoV-2 Alpha (B.1.1.7) variant, infection rates, antibody seroconversion and seroprevalence rates in secondary school students and staff: active prospective surveillance, December 2020 to March 2021, England (preprint)

Background In England, the rapid spread of the SARS-Cov-2 Alpha (B.1.1.7) variant from November 2020 led to national lockdown, including school closures in January 2021. We assessed SARS-CoV-2 infection, seroprevalence and seroconversion in students and staff when secondary schools reopened in March 2021. Methods Public Health England initiated SARS-CoV-2 surveillance in 18 secondary schools across six regions in September 2020. Participants provided nasal swabs for RT-PCR and blood samples for SARS-CoV-2 antibodies at the beginning (September 2020) and end (December 2020) of the autumn term and at the start of the spring term (March 2021). Findings In March 2021, 1895 participants (1100 students, 795 staff) were tested; 5.6% (61/1094) students and 4.4% (35/792) staff had laboratory-confirmed SARS-CoV-2 infection between December 2020 and March 2021. Nucleoprotein antibody seroprevalence was 36.3% (370/1018) in students and 31.9% (245/769) in staff, while spike protein antibody prevalence was 39.5% (402/1018) and 59.8% (459/769), respectively, similar to regional community seroprevalence. Between December 2020 and March 2021 (median 15.9 weeks), 14.8% (97/656; 95% CI: 12.2-17.7) students and 10.0% (59/590; 95% CI: 7.7-12.7) staff seroconverted. Weekly seroconversion rates were similar from September to December 2020 (8.0/1000) and from December 2020 to March 2021 (7.9/1000; students: 9.3/1,000; staff: 6.3/1,000). Interpretation By March 2021, a third of secondary school students and staff had serological evidence of prior infection based on N-antibody seropositivity, and an additional third of staff had evidence of vaccine-induced immunity based on S-antibody seropositivity. Further studies are needed to assess the impact of the Delta variant. Research in Context Evidence Before this study The Alpha variant is 30-70% more transmissible than previously circulating SARS-CoV-2 strains in adults and children. One outbreak investigation in childcare settings estimated similar secondary attack rates with the Alpha variant in children and adults. There are limited data on the impact of the Alpha variant in educational settings. In England, cases in primary and secondary school aged children increased rapidly from late November 2020 and peaked at the end of December 2020, leading to national lockdown including school closures. Added Value of This Study Seroconversion rates in staff and students during December 2020 to March 2021, when the Alpha variant was the primary circulating strain in England, were similar to the period between September 2020 and December 2020 when schools were fully open for in-person teaching. By March 2021, a third of students overall and more than half the students in some regions were seropositive for SARS-CoV-2 antibodies. Among staff, too, around a third had evidence of prior infection on serological testing and a further third had vaccine-induced immunity. Implications of all the Available Evidence SARS-CoV-2 antibody seroprevalence was high among secondary school students in March 2021 and is likely to be higher following the emergence of an even more transmissible Delta variant in May 2021. Education staff are increasingly being protected by the national COVID-19 immunisation programme. These findings have important implications for countries that are considering vaccination of children to control the pandemic

Children develop strong and sustained cross-reactive immune responses against spike protein following SARS-CoV-2 infection (preprint)

SARS-CoV-2 infection is generally mild or asymptomatic in children but the biological basis for this is unclear. We studied the profile of antibody and cellular immunity in children aged 3-11 years in comparison with adults. Antibody profiles in children were strong with high titres against spike protein and receptor binding domain (RBD). SARS-CoV-2 seroconversion in children strongly boosted antibody responses against seasonal beta-coronaviruses, partly through cross-recognition of the S2 domain, indicating a broad humoral response that was not seen in adults. T cell responses against spike were also >2-fold higher in children compared to adults and displayed a strong Th1 cytokine profile. SARS-CoV-2 spike-reactive cellular responses were present in more than half the seronegative children, indicating pre-existing cross-reactive responses or sensitization against SARS-CoV-2. Importantly, all children retained high antibody titres and cellular responses for more than 6 months after infection whilst relative antibody waning was seen in adults. Children thus distinctly generate robust, cross-reactive and sustained immune responses after SARS-CoV-2 infection with focussed specificity against spike protein. These observations demonstrate several novel features of SARS-CoV-2-specific immune responses in children and may provide insights into relative clinical protection in this group. Such information on the profile of natural infection will help to guide the introduction of vaccination regimens into the paediatric population.

Impact of COVID-19 Vaccination Program on Seroprevalence in Blood Donors in England, 2021 (preprint)

The COVID-19 vaccination programme commenced in the UK on 8th December 2020 primarily based on age; by 24 February 2021 approximately 93% of the English population aged 70-79 years had received at least 1 dose of either the Pfizer BioNTech or AstraZeneca vaccines. Using a nucleoprotein assay that detects antibodies following natural infection only and a spike assay that detects both infection and vaccine-induced responses, we aim to describe the impact of vaccination on SARS-CoV-2 antibody prevalence in English blood donors.

Characterising long COVID more than 6 months after acute infection in adults; prospective longitudinal cohort study, England (preprint)

Background Most individuals with COVID-19 will recover without sequelae, but some will develop long-term multi-system impairments. The definition, duration, prevalence and symptoms associated with long COVID, however, have not been established. Methods Public Health England (PHE) initiated longitudinal surveillance of clinical and non-clinical healthcare workers for monthly assessment and blood sampling for SARS-CoV-2 antibodies in March 2020. Eight months after enrolment, participants completed an online questionnaire including 72 symptoms in the preceding month. Symptomatic mild-to-moderate cases with confirmed COVID-19 were compared with asymptomatic, seronegative controls. Multivariable logistic regression was used to identify independent symptoms associated with long COVID. Findings All 2,147 participants were contacted and 1,671 (77.8%) completed the questionnaire, including 140 (8.4%) cases and 1,160 controls. At a median of 7.5 (IQR 7.1-7.8) months after infection, 20 cases (14.3%) had ongoing (4/140, 2.9%) or episodic (16/140, 11.4%) symptoms. We identified three clusters of symptoms associated with long COVID, those affecting the sensory (ageusia, anosmia, loss of appetite and blurred vision), neurological (forgetfulness, short-term memory loss and confusion/brain fog) and cardiorespiratory (chest tightness/pain, unusual fatigue, breathlessness after minimal exertion/at rest, palpitations) systems. The sensory cluster had the highest association with being a case (aOR 5.25, 95% CI 3.45-8.01). Dermatological, gynaecological, gastrointestinal or mental health symptoms were not significantly different between cases and controls. Interpretation Most persistent symptoms reported following mild COVID-19 were equally common in cases and controls. While all three clusters identified had a strong association with cases, the sensory cluster had the highest specificity and strength of association, and therefore, most likely to be characteristic of long COVID.

Seroprevalence of SARS-CoV-2 Antibodies in University Students: Cross-sectional Study, December 2020, England (preprint)

Background: In England, the reopening of universities in September 2020 coincided with a rapid increase in SARS-CoV-2 infection rates in university aged young adults. This study aimed to estimate SARS-CoV-2 antibody prevalence in students attending universities that had experienced a COVID-19 outbreak after reopening for the autumn term in September 2020.Methods: A cross-sectional serosurvey was conducted during 02-11 December 2020 in students aged ≤ 25 years across five universities in England. Blood samples for SARS-CoV-2 antibody testing were obtained using a self-sampling kit and analysed using the Abbott SARS-CoV-2 N antibody and/or an in-house receptor binding domain (RBD) assay. Findings: SARS-CoV-2 seroprevalence in 2,905 university students was 17.8% (95%CI, 16.5-19.3), ranging between 7.6%-29.7% across the five universities. Seropositivity was associated with being younger likely to represent first year undergraduates (aOR 3.2, 95% CI 2.0-4.9), living in halls of residence (aOR 2.1, 95% CI 1.7-2.7) and sharing a kitchen with an increasing number of students (shared with 4-7 individuals, aOR 1.43, 95%CI 1.12-1.82; shared with 8 or more individuals, aOR 1.53, 95% CI 1.04-2.24). Seropositivity was 49% in students living in halls of residence that reported high SARS-CoV-2 infection rates (>8%) during the autumn term.Interpretation: Despite large numbers of cases and outbreaks in universities, less than one in five students (17.8%) overall had SARS-CoV-2 antibodies at the end of the autumn term in England. In university halls of residence affected by a COVID-19 outbreak, however, nearly half the resident students became infected and developed SARS-CoV-2 antibodies.

Prospective Active National Surveillance of Preschools and Primary Schools for SARS-CoV-2 Infection and Transmission in England, June 2020: (sKIDs: COVID-19 Surveillance in School KIDs) (preprint)

Background: Many countries re-opened schools after national lockdown but little is known about the risk of SARS-CoV-2 infection and transmission in educational settings. Public Health England conducted six-month prospective surveillance in primary schools across England. Methods: The COVID-19 Surveillance in School KIDs (sKIDs) study included two arms: weekly nasal swabs for ≥4 weeks following partial reopening during the summer half-term (June to mid-July 2020) and blood sampling with nasal and throat swabs at the beginning and end of the summer half-term, and, following full reopening in September 2020, at the end of the autumn term (end-November 2020). Results: In round 1, 12,026 participants (59.1% students, 40.9% staff) in 131 schools had 43,091 swabs taken. Weekly SARS-CoV-2 infection rates were 3.9 (1/25,537; 95% CI, 0.10-21.8) and 11.3 (2/17,554; 95% CI, 1.4-41.2) per 100,000 students and staff. At recruitment, N-antibody positivity in 45 schools was 11.1% (91/817; 95%CI, 9.2-13.5%) in students and 15.1% (209/1381; 95%CI, 13.3-17.1%) in staff, similar to local community seroprevalence. Seropositivity was not associated with school attendance during lockdown or staff contact with students. Round 2 participation was 73.7% (1,619/2,198) and only five (4 students, 1 staff) seroconverted. In round 3, when 61.9% (1,361/2,198) of round 1 participants were re-tested, seroconversion rates were 3.4% (19/562; 95%CI, 2.0-5.2) in students and 3.9% (36/930; 95%CI, 2.7-5.3) in staff. Conclusions: SARS-CoV-2 infection rates, assessed using nasal swabs for acute infection and serum antibodies for prior infection, were low following partial and full reopening of primary schools in England.Funding Statement: This surveillance was funded by the Department of Health and Social Care (DHSC).Declaration of Interests: None to declare.Ethics Approval Statement: The surveillance protocol was approved by the Public Health England Research Ethics Governance Group (R&D REGG Ref: NR0209, 16 May 2020).

Robust SARS-CoV-2-specific T-cell immunity is maintained at 6 months following primary infection (preprint)

The immune response to SARS-CoV-2 is critical in both controlling primary infection and preventing re-infection. However, there is concern that immune responses following natural infection may not be sustained and that this may predispose to recurrent infection. We analysed the magnitude and phenotype of the SARS-CoV-2 cellular immune response in 100 donors at six months following primary infection and related this to the profile of antibody level against spike, nucleoprotein and RBD over the previous six months. T-cell immune responses to SARS-CoV-2 were present by ELISPOT or ICS analysis in all donors and are characterised by predominant CD4+ T cell responses with strong IL-2 cytokine expression. Median T-cell responses were 50% higher in donors who had experienced an initial symptomatic infection indicating that the severity of primary infection establishes a set-point for cellular immunity that lasts for at least 6 months. The T-cell responses to both spike and nucleoprotein/membrane proteins were strongly correlated with the peak antibody level against each protein. The rate of decline in antibody level varied between individuals and higher levels of nucleoprotein-specific T cells were associated with preservation of NP-specific antibody level although no such correlation was observed in relation to spike-specific responses. In conclusion, our data are reassuring that functional SARS-CoV-2-specific T-cell responses are retained at six months following infection although the magnitude of this response is related to the clinical features of primary infection.

Robust SARS-CoV-2-specific T-cell immunity is maintained at 6 months following primary infection (preprint)

The immune response to SARS-CoV-2 is critical in both controlling primary infection and preventing re-infection. However, there is concern that immune responses following natural infection may not be sustained and that this may predispose to recurrent infection. We analysed the magnitude and phenotype of the SARS-CoV-2 cellular immune response in 100 donors at six months following primary infection and related this to the profile of antibody level against spike, nucleoprotein and RBD over the previous six months. T-cell immune responses to SARS-CoV-2 were present by ELISPOT or ICS analysis in all donors and are characterised by predominant CD4+ T cell responses with strong IL-2 cytokine expression. Median T-cell responses were 50% higher in donors who had experienced an initial symptomatic infection indicating that the severity of primary infection establishes a ‘setpoint’ for cellular immunity that lasts for at least 6 months. The T-cell responses to both spike and nucleoprotein/membrane proteins were strongly correlated with the peak antibody level against each protein. The rate of decline in antibody level varied between individuals and higher levels of nucleoprotein-specific T cells were associated with preservation of NP-specific antibody level although no such correlation was observed in relation to spike-specific responses. In conclusion, our data are reassuring that functional SARS-CoV-2-specific T-cell responses are retained at six months following infection although the magnitude of this response is related to the clinical features of primary infection.

Serological surveillance of SARS-CoV-2: trends and humoral response in a cohort of public health workers (preprint)

Background There is considerable debate about the rate of antibody waning after SARS-CoV-2 infection, raising questions around long-term immunity following both natural infection and vaccination. We undertook prospective serosurveillance in a large cohort of healthy adults from the start of the epidemic in England. Methods The serosurveillance cohort included office and laboratory-based staff and healthcare workers in 4 sites in England, who were tested monthly for SARS-CoV-2 spike protein and nucleoprotein IgG between 23rd March and 20th August 2020. Antibody levels from 21 days after a positive test were modelled using mixed effects regression models. Findings In total, 2247 individuals were recruited and 2014 (90%) had 3-5 monthly antibody tests. Overall, 272 (12.1%) of individuals had at least one positive/equivocal spike protein IgG result, with the highest proportion in a hospital site (22%), 14% in London and 2.1% in a rural area. Results were similar for nucleoprotein IgG. Following a positive result, 39/587 (6.6%) tested negative for nucleoprotein IgG and 52/515 (10.1%) for spike protein IgG. Nucleoprotein IgG declined by 6.4% per week (95% CI, 5.5-7.4%; half-life, 75 [95% CI, 66-89] days) and spike protein IgG by 5.8% (95% CI, 5.1-6.6%; half-life, 83 [95% CI, 73-96] days). Conclusions Over the study period SARS-CoV-2 seropositivity was 8-10% overall and up to 21% in clinical healthcare workers. In seropositive individuals, nucleoprotein and spike protein IgG antibodies declined with time after infection and 50% are predicted to fall below the positive test threshold after 6 months.

Use of dried blood spot samples for SARS-CoV-2 antibody detection using the Roche Elecsys high throughput immunoassay (preprint)

BackgroundDried blood spot samples (DBS) provide an alternative sample type to venous blood samples for antibody testing. DBS are used by NHS for diagnosing HCV and by PHE for large scale HIV and Hepatitis C serosurveillance; the applicability of DBS based approaches to SARS-CoV-2 antibody detection is uncertain. ObjectiveTo compare antibody detection in dried blood spot eluates using the Roche Elecsys (R) immunoassay (index test) with antibody detection in paired plasma samples, using the same assay (reference test). SettingOne Police and one Fire & Rescue facility in England. Participants195 participants within a larger sample COVID-19 serodiagnostics study of keyworkers, EDSAB-HOME. Outcome MeasuresSensitivity and specificity of DBS (the index test) relative to plasma (the reference test), at an experimental cut-off; quality of DBS sample collected; estimates of relative sensitivity of DBS vs. plasma immunoassay in a larger population. Results18/195 (9.2%) participants tested positive using plasma samples. DBS sample quality varied markedly by phlebotomist, and low sample volume significantly reduced immunoassay signals. Using a cut-off of ten median absolute deviations above the immunoassay result with negative samples, sensitivity and specificity of DBS were 89.0% (95% CI 67.2, 96.9%) and 100.0% (95% CI 97.9, 100%) respectively compared with using plasma. The limit of detection for DBS is about 30 times higher than for plasma. ConclusionDBS use for SARS-CoV-2 serology, though feasible, is insensitive relative to immunoassays on plasma. Sample quality impacts on assay performance. Alternatives, including the collection of capillary blood samples, should be considered for screening programs.

Self assessment overestimates historical COVID-19 disease relative to sensitive serological assays: cross sectional study in UK key workers (preprint)

Objective To measure the association between self-reported signs and symptoms and SARS-CoV-2 seropositivity. Design Cross sectional study of three key worker groups. Setting Six acute NHS hospitals and two Police and Fire and Rescue sites in England. Participants Individuals were recruited from three streams: (A) Police and Fire and Rescue services (n=1147), (B) healthcare workers (n=1546) and (C) healthcare workers with previously positive virus detection (n=154). Main outcome measures Detection of anti-SARS-CoV-2 antibodies in plasma. Results 943 of the 2847 participants (33%) reported belief they had had COVID-19, having experienced compatible symptoms (including 152 from Stream C). Among individuals reporting COVID-19 compatible symptoms, 466 (49%) were seronegative on both Nucleoprotein (Roche) and Spike-protein (EUROIMMUN) antibody assays. However, among the 268 individuals with prior positive SARS-CoV-2 tests, of whom 96% reported symptoms with onset a median of 63 days (IQR 52 to 75 days) prior to venesection, Roche and EUROIMMUN assays had 96.6% (95% CI 93.7% to 98.2%) and 93.3% (95% CI 89.6% to 95.7%) sensitivity respectively. Symptomatic but seronegative individuals had significantly earlier symptom onset dates than the symptomatic seropositive individuals, shorter illness duration and a much lower anosmia reporting frequency. Conclusions Self-reported belief of COVID-19 was common among our frontline worker cohort. About half of these individuals were seronegative, despite a high sensitivity of serology in this cohort, at least in individuals with previous positive PCR results. This is compatible with non-COVID-19 respiratory disease during the COVID-19 outbreak having been commonly mistaken for COVID-19 within the key worker cohort studied.