Reduced risk of SARS-CoV-2 infection among household contacts with recent vaccination and past COVID-19 infection: results from two multi-site case-ascertained household transmission studies (preprint)

Background COVID-19 vaccines reduce the risk of severe disease, but it is less clear what effect vaccines have on reducing the risk of infection in high contact settings like households, alone or in combination with prior infection. Methods Households with an individual who tested positive for SARS-CoV-2 during Sep 2021-May 2023 were screened nationwide and at 7 sentinel sites and enrolled if the index cases illness onset was [≤]6 days prior. Household members had daily self-collected nasal swabs tested by RT-PCR for SARS-CoV-2. COVID-19 vaccination status was assessed by plausible self-report (with date) or vaccination records. Prior infection was assessed by self-reported prior testing and by anti-nucleocapsid antibodies presence at enrollment. The effects of prior immunity, including vaccination, prior infection, or hybrid immunity (both vaccination and prior infection) on SARS-CoV-2 infection risk among household contacts were assessed by robust, clustered multivariable Poisson regression. Findings There were 1,532 contacts from 905 households included in this analysis. Of these, 67% were enrolled May-November 2022, when Omicron BA.4/5 predominated. Most contacts (89%) had some immunity to SARS-CoV-2 at the time of household exposure: 8% had immunity from prior infection alone, 51% from vaccination alone, and 29% had hybrid immunity. Sixty percent of contacts tested SARS-CoV-2-positive during follow-up. The risk of SARS-CoV-2 infection was not significantly reduced by vaccination but was reduced among those with prior infection considering such immunity separately (adjusted relative risk 0.83; 95% confidence interval: 0.77, 0.90); however, when accounting for both sources of immunity, only contacts with vaccination and prior infection had significantly reduced risk of infection (aRR: 0.81, 95% CI: 0.70, 0.93). The risk of infection was lower when the last immunizing event (vaccination or infection) occurred [≤]6 months before COVID-19 affected the household (aRR: 0.69, 95% CI: 0.57, 0.83). Interpretation Immunity from COVID-19 vaccination and prior infection was synergistic in protecting household contacts from SARS-CoV-2 infection. These data support COVID-19 vaccination, even for those who have been previously infected.

Work Attendance during Acute Respiratory Illness Before and During the COVID-19 Pandemic, United States, 2018-2022 (preprint)

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) and influenza viruses can be transmitted by infected persons who are pre-symptomatic or symptomatic. To assess impact of the COVID-19 pandemic on work attendance during illness, we analyzed prospectively collected data from persons with acute respiratory illness (ARI) enrolled in a multi-state study during 2018-2022. Persons with prior experience working from home were significantly less likely than those without this experience to work onsite on the day before illness and during the first 3 days of illness; the effect was more pronounced for the COVID-19 pandemic period than the pre-pandemic influenza seasons. Persons with influenza or COVID-19 were significantly less likely to work onsite than persons with other ARIs. Among persons for whom positive COVID-19 test results were available by the second or third day of illness, few worked onsite. Work-from-home policies may reduce the likelihood of workplace exposures to respiratory viruses.

Changes in Transmission and Symptoms of SARS-CoV-2 in United States Households, April 2020-September 2022 (preprint)

BackgroundThe natural history of SARS-CoV-2 infection and transmission dynamics may have changed as SARS-CoV-2 has evolved and population immunity has shifted. MethodsHousehold contacts, enrolled from two multi-site case-ascertained household transmission studies (April 2020-April 2021 and September 2021-September 2022), were followed for 10-14 days after enrollment with daily collection of nasal swabs and/or saliva for SARS-CoV-2 testing and symptom diaries. SARS-CoV-2 virus lineage was determined by whole genome sequencing, with multiple imputation where sequences could not be recovered. Adjusted infection risks were estimated using modified Poisson regression. Findings858 primary cases with 1473 household contacts were examined. Among unvaccinated household contacts, the infection risk adjusted for presence of prior infection and age was 58% (95% confidence interval [CI]: 49-68%) in households currently exposed to pre-Delta lineages and 90% (95% CI: 74-100%) among those exposed to Omicron BA.5 (detected May - September 2022). The fraction of infected household contacts reporting any symptom was similarly high between pre-Delta (86%, 95% CI: 81-91%) and Omicron lineages (77%, 70-85%). Among Omicron BA.5-infected contacts, 48% (41-56%) reported fever, 63% (56-71%) cough, 22% (17-28%) shortness of breath, and 20% (15-27%) loss of/change in taste/smell. InterpretationThe risk of infection among household contacts exposed to SARS-CoV-2 is high and increasing with more recent SARS-CoV-2 lineages. This high infection risk highlights the importance of vaccination to prevent severe disease. FundingFunded by the Centers for Disease Control and Prevention and the Food and Drug Administration. Key points- Monitoring the transmissibility and symptomatology of SARS-CoV-2 lineages is important for informing public health practice and understanding the epidemiology of COVID-19; household transmission studies contribute to our understanding of the natural history of SARS-CoV-2 infections and the transmissibility of SARS-CoV-2 variants. - The Omicron BA.5 sub-lineage is highly transmissible, similar to previous Omicron sub-lineages. - Over 80% of infected household contacts reported at least 1 symptom during their infection and the proportion of household contacts with asymptomatic infection did not differ by SARS-CoV-2 variant. The most common symptom was cough. Change in taste or smell was more common in Omicron BA.5 infections, compared to previous Omicron sub-lineages, but less common compared to pre-Delta lineages. - The high infection risk among household contacts supports the recommendations that individuals maintain up-to-date and lineage-specific vaccinations to mitigate further risks of severe disease.

Effectiveness of COVID-19 mRNA vaccine booster dose relative to primary series during a period of Omicron circulation (preprint)

During a period of Omicron variant circulation, we estimated relative VE of COVID-19 mRNA booster vaccination versus primary two-dose series in an ongoing community cohort. Relative VE was 66% (95% CI: 46%, 79%) favoring the booster dose compared to primary series vaccination. Our results support current booster recommendations.

Effectiveness of 2 and 3 mRNA COVID-19 Vaccines Doses against Omicron and Delta-Related Outpatient Illness among Adults, October 2021 - February 2022 (preprint)

Background: We estimated SARS-CoV-2 Delta and Omicron-specific effectiveness of 2 and 3 mRNA COVID-19 vaccine doses in adults against symptomatic illness in US outpatient settings. Methods: Between October 1, 2021, and February 12, 2022, research staff consented and enrolled eligible participants who had fever, cough, or loss of taste or smell and sought outpatient medical care or clinical SARS-CoV-2 testing within 10 days of illness onset. Using the test-negative design, we compared the odds of receiving 2 or 3 mRNA COVID-19 vaccine doses among SARS-CoV-2 cases versus controls using logistic regression. Regression models were adjusted for study site, age, onset week, and prior SARS-CoV-2 infection. Vaccine effectiveness (VE) was calculated as (1-adjusted odds ratio) x 100%. Results: Among 3847 participants included for analysis, 574 (32%) of 1775 tested positive for SARS-CoV-2 during the Delta predominant period and 1006 (56%) of 1794 participants tested positive during the Omicron predominant period. When Delta predominated, VE against symptomatic illness in outpatient settings was 63% (95% CI: 51% to 72%) among mRNA 2-dose recipients and 96% (95% CI: 93% to 98%) for 3-dose recipients. When Omicron predominated, VE was 21% (95% CI: -6% to 41%) among 2-dose recipients and 62% (95% CI: 48% to 72%) among 3-dose recipients. Conclusions: In this adult population, 3 mRNA COVID-19 vaccine doses provided substantial protection against symptomatic illness in outpatient settings when the Omicron variant became the predominant cause of COVID-19 in the U.S. These findings support the recommendation for a 3rd mRNA COVID-19 vaccine dose.

SARS-CoV-2 virus dynamics in recently infected people - data from a household transmission study (preprint)

We used daily real-time reverse-transcription polymerase chain reaction (rRT-PCR) results from 67 cases of SARS-CoV-2 infection in a household transmission study to examine the trajectory of cycle threshold (Ct) values, an inverse correlate of viral RNA concentration, from nasal specimens collected between April 2020 and May 2021. Ct values varied over the course of infection, across RT-PCR platforms, and by participant age. Specimens collected from children and adolescents showed higher Ct values and adults aged [≥]50 years showed lower Ct values than adults aged 18-49 years. Ct values were lower on days when participants reported experiencing symptoms.

Vaccine Effectiveness against COVID-19 among Symptomatic Persons Aged >=12 Years with Reported Contact with COVID-19 Cases, February - September 2021 (preprint)

Individuals in contact with persons with COVID-19 are at high risk of developing COVID-19, but protection offered by COVID-19 vaccines in the context of known exposure is unknown. Symptomatic outpatients reporting acute onset of COVID-19-like illness and tested for SARS-CoV-2 infection were enrolled. Among 2,229 participants, 283/451 (63%) of those reporting contact and 331/1778 (19%) without known contact tested SARS-CoV-2 positive. Using the test-negative design, adjusted vaccine effectiveness was 71% (95% confidence interval, 49%-83%) among fully vaccinated participants reporting contact versus 80% (95% CI, 72%-86%) among those without. This study supports COVID-19 vaccination and highlights the importance of efforts to increase vaccination coverage.

mRNA COVID-19 vaccine effectiveness against SARS-CoV-2 infection in a prospective community cohort, rural Wisconsin, November 2020-December 2021 (preprint)

Reduced COVID-19 vaccine effectiveness (VE) has been observed with increasing predominance of the Delta variant. In a prospective rural community cohort of 1265 participants, VE against symptomatic and asymptomatic SARS-CoV-2 infection was 56% for mRNA COVID-19 vaccines.

Household Transmission and Clinical Features of SARS-CoV-2 Infections by Age in 2 US Communities (preprint)

OBJECTIVESExamine age differences in SARS-CoV-2 transmission risk from primary cases and infection risk among household contacts, and symptoms among those with SARS-CoV-2 infection. METHODSPeople with SARS-CoV-2 infection in Nashville, Tennessee and central and western Wisconsin and their household contacts were followed daily for 14 days to ascertain symptoms and secondary transmission events. Households were enrolled between April 2020 and April 2021. Secondary infection risks (SIR) by age of the primary case and contacts were estimated using generalized estimating equations. RESULTSThe 226 primary cases were followed by 198 (49%) secondary SARS-CoV-2 infections among 404 household contacts. Age group-specific SIR among contacts ranged from 36% to 53%, with no differences by age. SIR was lower from primary cases aged 12-17 years than from primary cases 18-49 years (risk ratio [RR] 0.42; 95% confidence interval [CI] 0.19-0.91). SIR was 55% and 45%, respectively, among primary case-contact pairs in the same versus different age group (RR 1.47; 95% CI 0.98-2.22). SIR was highest among primary case-contacts pairs aged [≥]65 years (76%) and 5-11 years (69%). Among secondary SARS-CoV-2 infections, 19% were asymptomatic; there was no difference in the frequency of asymptomatic infections by age group. CONCLUSIONSBoth children and adults can transmit and are susceptible to SARS-CoV-2 infection. SIR did not vary by age, but further research is needed to understand age-related differences in probability of transmission from primary cases by age.

mRNA Vaccine Effectiveness against COVID-19 among Symptomatic Outpatients Aged >=16 Years in the United States, February to May 2021 (preprint)

Evaluations of vaccine effectiveness (VE) are important to monitor as COVID-19 vaccines are introduced in the general population. Research staff enrolled symptomatic participants seeking outpatient medical care for COVID-19-like illness or SARS-CoV-2 testing from a multisite network. VE was evaluated using the test-negative design. Among 236 SARS-CoV-2 nucleic acid amplification test-positive and 576 test-negative participants aged [≥]16 years, VE of mRNA vaccines against COVID-19 was 91% (95% CI: 83-95) for full vaccination and 75% (95% CI: 55-87) for partial vaccination. Vaccination was associated with prevention of most COVID-19 cases among people seeking outpatient medical care.