Hybrid immunity improves the immune response after fourth Covid-19 vaccine dose in individuals with medical conditions predisposing to severe Covid-19 (preprint)

BackgroundData on Covid-19 booster vaccinations and subsequent infections on immune responses in the immunocompromised is limited. We studied antibody responses after the fourth dose and subsequent breakthrough infection to define patient groups benefiting most from boosters. MethodsIn Finland, fourth vaccine (booster) doses were first recommended for severely immunocompromised individuals, whom we invited to participate in 2022. We assessed spike protein specific IgG antibody levels and neutralizing antibodies (NAb) against the ancestral and Omicron BA.1 strains one month after the fourth dose from 488 adult participants and compared to the levels of 35 healthy controls after 3 doses. We used Bayesian generalized linear modelling to assess factors explaining antibody concentrations after the fourth dose. We assessed vaccine-induced and hybrid immunity six months after the last vaccine dose. ResultsChronic kidney disease (CKD) and immunosuppressive therapy (IT) were identified as factors explaining sub-optimal antibody responses. The proportion of participants with a normal antibody response and NAbs were significantly lower in CKD patients as compared to controls. By the 6-month sampling one third of the participants became infected, which enhanced antibody levels notably in most immunocompromised participants. ConclusionsImpaired antibody responses, especially NAbs against the Omicron lineage, predict limited protection in individuals with CKD, and highlight the need for alternative pharmaceutical preventive strategies. Vaccination strategies should take into account development of robust hybrid immunity responses also among the immunocompromised.

Changes in SARS-CoV-2 seroprevalence and population immunity in Finland, 2020-2022 (preprint)

Studying the prevalence of SARS-CoV-2 specific antibodies (seroprevalence) allows assessing the impact of epidemic containment measures and vaccinations, as well as estimation of the number of infections regardless of viral testing. We assessed antibody-mediated immunity to SARS-CoV-2 induced by infections and vaccinations from April 2020 to December 2022 in Finland by measuring serum IgG to SARS-CoV-2 nucleoprotein (N-IgG) and spike glycoprotein from randomly selected 18-85-year-old subjects (n=9794). N-IgG seroprevalence remained at <7% until the last quartile (Q) of 2021. After the emergence of the omicron variant, N-IgG seroprevalence increased rapidly and was 31% in Q1/2022 and 54% in Q4/2022. Seroprevalence was highest in youngest age groups from Q2/2022 onwards. We estimated that 51% of the Finnish 18-85-year-old population had antibody-mediated hybrid immunity induced by a combination of vaccinations and infections by the end of 2022. In conclusion, major shifts in the COVID-19 pandemic and resulting population immunity could be observed by serological testing.

Strong neutralizing antibody responses to SARS-CoV-2 variants following a single vaccine dose in subjects with previous SARS-CoV-2 infection (preprint)

Background Previous SARS-CoV-2 infection primes the immune system and thus individuals who recovered from infection have enhanced immune responses to subsequent vaccination (hybrid immunity). However, it remains unclear how well hybrid immunity induced by severe or mild infection can cross-neutralize emerging variants. We aimed to compare the strength and breadth of antibody responses in vaccinated recovered and uninfected subjects. Methods We measured spike-specific IgG and neutralizing antibodies (NAbs) from vaccinated subjects including 320 with hybrid immunity and 20 without previous infection. From 29 subjects with a previous severe or mild infection, we also measured NAb responses against Alpha (B.1.1.7), Beta (B.1.351), Delta (B.1.617.2) and Omicron (B.1.1.529/BA.1) variants following vaccination. Results A single vaccine dose induced 2-fold higher anti-spike IgG concentrations and 3-fold higher neutralizing potency of antibodies in previously infected compared to uninfected fully vaccinated subjects. We found similar IgG concentrations in previously infected subjects after one or two vaccine doses. NAb titers were higher in subjects with severe compared to those with mild infection. This difference remained after vaccination with sequentially decreasing titers against Alpha, Beta, Delta, and Omicron variants. Conclusions Hybrid immunity induced strong IgG responses, particularly after severe infection. However, the NAb titers were low against heterologous variants, especially against Omicron.

Neutralizing antibodies to SARS-CoV-2 Omicron variant after 3rd mRNA vaccination in health care workers and elderly subjects and response to a single dose in previously infected adults (preprint)

The emergence of SARS-CoV-2 Omicron variant (B.1.1.529) with major spike protein mutations has raised concern over potential neutralization escape and breakthrough infections among vaccinated and previously SARS-CoV-2 infected subjects. We measured cross-protective antibodies against variants in health care workers (HCW, n=20) and nursing home residents (n=9) from samples collected 1-2 months following the booster (3rd) dose. We also assessed the antibody responses in prior to Omicron era infected subjects (n=38) with subsequent administration of a single mRNA vaccine dose. Following booster vaccination HCWs had high IgG antibody concentrations to the spike protein and neutralizing antibodies (NAb) were detectable against all variants. IgG concentrations among the elderly remained lower, and some lacked NAbs against the Beta and Omicron variants. NAb titers were significantly reduced against Delta, Beta and Omicron compared to wild-type virus regardless of age. Vaccination induced high IgG concentrations and variable titers of cross-reactive NAbs in previously infected subjects, whereas NAb titers against Omicron were barely detectable 1-month post-infection. High IgG concentrations with cross-protective neutralizing activity were detected after three COVID-19 vaccine doses in HCWs. However, lower NAb titers seen in the frail elderly suggest inadequate protection against Omicron breakthrough infections, yet protection against severe COVID-19 is expected. Clinical trial registration EudraCT 2021-004788-29

A highly sensitive and specific SARS-CoV-2 spike- and nucleoprotein-based fluorescent multiplex immunoassay (FMIA) to measure IgG, IgA and IgM class antibodies (preprint)

Background. Validation and standardization of accurate serological assays are crucial for the surveillance of the coronavirus disease 2019 (COVID-19) pandemic and population immunity. Methods. We describe the analytical and clinical performance of an in-house fluorescent multiplex immunoassay (FMIA) for simultaneous quantification of antibodies against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleoprotein and spike glycoprotein. Furthermore, we calibrated IgG-FMIA against World Health Organisation (WHO) International Standard and compared FMIA results to an in-house enzyme immunoassay (EIA) and a microneutralisation test (MNT). We also compared the MNT results of two laboratories. Results. IgG-FMIA displayed 100% specificity and sensitivity for samples collected 13-150 days post-onset of symptoms (DPO). For IgA- and IgM-FMIA 100% specificity and sensitivity were obtained for a shorter time window (13-36 and 13-28 DPO for IgA- and IgM-FMIA, respectively). FMIA and EIA results displayed moderate to strong correlation, but FMIA was overall more specific and sensitive. IgG-FMIA identified 100% of samples with neutralising antibodies (NAbs). Anti-spike IgG concentrations correlated strongly ({rho}=0.77-0.84, P<2.2x10-16) with NAb titers. The NAb titers of the two laboratories displayed a very strong correlation ({rho}=0.95, P<2.2x10-16). Discussion. Our results indicate good correlation and concordance of antibody concentrations measured with different types of in-house SARS-CoV-2 antibody assays. Calibration against WHO international standard did not, however, improve the comparability of FMIA and EIA results. keywords: SARS-CoV-2, COVID-19, antibody, immunoassay, nucleoprotein, spike glycoprotein, WHO international standard, neutralising antibodies, microneutralisation, receptor-binding domain

High secondary attack rate and persistence of SARS-CoV-2 antibodies in household transmission study participants, Finland 2020 (preprint)

Background Household transmission studies offer the opportunity to assess both secondary attack rate and persistence of SARS-CoV-2 antibodies over time. Methods We invited confirmed COVID-19 cases and their household members to attend up to four household visits with collection of nasopharyngeal and serum samples over 28 days after index case onset. We calculated secondary attack rates (SAR) based on the presence of SARS-CoV-2 nucleoprotein IgG antibodies (IgG Ab) and/or neutralizing antibodies (NAb) overall and per households. Three and six months later, we assessed the persistence of SARS-CoV-2 antibodies. Findings We recruited 39 index cases and 90 household members. Among 87 household members evaluated, SAR was 48% (n=42), including 37 symptomatic secondary cases. In total, 80/129 (62%) participants developed both IgG Ab and NAb, while three participants only developed IgG Ab. Among participants who had both IgG Ab and NAb during the initial follow-up, 68/69 (99%) and 63/70 (90%) had IgG Ab and NAb at 3 months, while at 6 months, 59/75 (79%) and 63/75 (84%) had IgG Ab and NAb, respectively. Participants who required hospital care had initially 5-fold IgG Ab concentrations compared to cases with mild symptoms and 8-fold compared to asymptomatic cases. Interpretation Following detection of a COVID-19 case in a household, other members had a high risk of becoming infected. Follow-up of participants showed strong persistence of antibodies in most cases. Funding This study was supported by THL coordinated funding for COVID-19 research (Finnish Government's supplementary budget) and by the Academy of Finland (Decision number 336431).

High Secondary Attack Rate and Persistence of SARS-CoV-2 Antibodies in Household Transmission Study Participants, Finland 2020 (preprint)

Background: Household transmission studies offer the opportunity to assess both secondary attack rate and persistence of SARS-CoV-2 antibodies over time.Methods: We invited confirmed COVID-19 cases and their household members to attend up to four household visits with collection of nasopharyngeal and serum samples over 28 days after index case onset. We calculated secondary attack rates (SAR) based on the presence of SARS-CoV-2 nucleoprotein IgG antibodies (IgG Ab) and/or neutralizing antibodies (NAb) overall and per households. Three and six months later, we assessed the persistence of SARS-CoV-2 antibodies.Findings: We recruited 39 index cases and 90 household members. Among 87 household members evaluated, SAR was 48% (n=42), including 37 symptomatic secondary cases. In total, 80/129 (62%) participants developed both IgG Ab and NAb, while three participants only developed IgG Ab. Among participants who had both IgG Ab and NAb during the initial follow-up, 68/69 (99%) and 63/70 (90%) had IgG Ab and NAb at 3 months, while at 6 months, 59/75 (79%) and 63/75 (84%) had IgG Ab and NAb, respectively. Participants who required hospital care had initially 5-fold IgG Ab concentrations compared to cases with mild symptoms and 8-fold compared to asymptomatic cases.Interpretation: Following detection of a COVID-19 case in a household, other members had a high risk of becoming infected. Follow-up of participants showed strong persistence of antibodies in most cases.Funding Information: This study was supported by THL coordinated funding for COVID-19 research (Finnish Government’s supplementary budget) and by the Academy of Finland (Decision number 336431).Declaration of Interests: THL has received research funding for studies not related to COVID-19 from GlaxoSmithKline Vaccines (NE, CV, AAP and MM as investigators), Pfizer (AAP) and Sanofi Pasteur (AAP). Other authors have no competing interests.Ethics Approval Statement: The Finnish communicable diseases law and the law on the duties of the Finnish Institute for Health and Welfare allowed the implementation of the initial household transmission study and 3 months convalescent sample collection without seeking further institutional ethical review11,12. The protocol for the follow-up visits at 6 months was reviewed and approved by the HUS ethical committee and registered under the Development of seroprevalence in Finland during the new coronavirus (SARS-CoV-2) epidemic – serological population study protocol HUS/1137/2020. Informed consent was obtained from all cases and contacts or their parents or legal guardian before any procedure was performed.

Persistence of neutralizing antibodies a year after SARS-CoV-2 infection (preprint)

Understanding for how long antibodies persist following Severe acute respiratory coronavirus 2 (SARS-CoV-2) infection provides important insight into estimating the duration of immunity induced by infection. We assessed the persistence of serum antibodies following wild-type SARS-CoV-2 infection six and twelve months after diagnosis in 367 individuals of whom 13% had severe disease requiring hospitalization. We determined the SARS-CoV-2 spike (S-IgG) and nucleoprotein IgG concentrations and the proportion of subjects with neutralizing antibodies (NAb). We also measured the NAb titers among a smaller subset of participants (n=78) against a wild-type virus (B.1) and three variants of concern (VOCs): Alpha (B.1.1.7), Beta (B.1.351) and Delta (B.1.617.2). We found that NAb against the wild-type virus and S-IgG persisted in 89% and 97% of subjects for at least twelve months after infection, respectively. IgG and NAb levels were higher after severe infection. NAb titers were significantly lower against variants compared to the wild-type virus.

Analytical and clinical evaluation of antibody tests for SARS-CoV-2 serosurveillance studies used in Finland in 2020 (preprint)

Background: Sensitive and highly specific antibody tests are critical for detection of SARS-CoV-2 antibodies especially in populations where seroprevalence is low. Aim: To set up, optimize and evaluate the analytical and clinical performance of a new in-house microsphere immunoassay for measurement of IgG antibodies to SARS-CoV-2 nucleoprotein for assessment of population seroprevalence in Finland. Methods: We set up a new in-house microsphere immunoassay (FMIA) with SARS-CoV-2 nucleoprotein and optimized its analytical performance. For evaluation of clinical performance, we tested sera collected in a well-characterized cohort of PCR positive-confirmed SARS-CoV-2 patients (n=89) with mostly mild symptoms, and before the COVID-19 pandemic (n=402), for nucleoprotein specific IgG concentrations by FMIA and a commercial chemiluminescent immunoassay and for neutralizing antibodies by the microneutralization test. Results: The analytical performance of FMIA was established in terms of sensitivity, linearity and precision. FMIA discriminated between COVID-19 patient and control samples with high specificity (100%) and sensitivity (100%). We generated FMIA seropositivity cut-offs, 0.46 and 1.71 U/ml, for low- and high-seroprevalence settings, respectively. In addition, we obtained high level of agreement between FMIA results and results by the microneutralization test. Conclusion: The fluorescent microsphere immunoassay showed excellent analytical and clinical performance and is well suited for serosurveillance studies of SARS-CoV-2. However, to optimize analytical sensitivity and clinical specificity of the assay, different seropositivity thresholds depending on the intended use of the assay and the target population, may be needed.