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Effectiveness of COVID-19 vaccination against SARS-CoV-2 Infection among Residents of US Nursing Homes, Before and During the Delta variant Predominance, December 2020 – November 2021

Abstract Background Residents of nursing homes experience disproportionate morbidity and mortality related to COVID-19 and were prioritized for vaccine introduction. We evaluated COVID-19 vaccine effectiveness (VE) in preventing SARS-CoV-2 infections among nursing home residents. Methods We used a retrospective cohort of 4,315 nursing home residents during December 14, 2020 - November 9, 2021. A Cox proportional hazards model was used to estimate hazard ratios comparing residents with a completed vaccination series to unvaccinated among those with and without prior SARS-CoV-2 infection (identified using positive SARS-CoV-2 tests and/or diagnosis codes), by vaccine product, and by period (before and during Delta variant predominance). VE was estimated as one minus the hazard ratio times 100%. Results Overall adjusted VE for the completed vaccination series was 58% (95%CI: 44%, 69%) among residents without a history of SARS-CoV-2 infection. During the pre-Delta period, the VE within 150 days of receipt of the second dose of Pfizer-BioNTech (67%, 95%CI: 40%, 82%) and Moderna (75%, 95%CI: 32%, 91%) was similar. During the Delta period, VE measured >150 days after the second dose was 33% (95%CI: -2%, 56%) for Pfizer-BioNTech and 77% (95%CI: 48%, 91%) for Moderna. Rates of infection were 78% lower (95%CI: 67%, 85%) among residents with prior SARS-CoV-2 infection and completed vaccination series compared to unvaccinated residents without a history of SARS-CoV-2 infection. Conclusions COVID-19 vaccines were effective in preventing SARS-CoV-2 infections among nursing home residents and history of prior SARS-CoV-2 infection provided additional protection. Maintaining high coverage of recommended doses of COVID-19 vaccines remains a critical tool for preventing infections in nursing homes.

Nursing home residents were not included in clinical trials evaluating efficacy of 12 COVID-19 vaccines. [3,4] However, several observational studies evaluated vaccine 13 effectiveness (VE) among nursing home residents using real-world data and demonstrated that 14 COVID-19 vaccines are highly effective in preventing infection, hospitalization, and mortality in 15 these populations. [5][6][7][8] While a decline in VE during the time period where the Delta variant 16 (B1.617.2) was circulating has been documented in this population, [9] current studies have not 17 been able to assess the relative contribution of waning vaccine-induced immunity since receipt of 18 a primary vaccination series or the predominance of emerging variants to observed changes in 19 VE over time. 20 There is accruing evidence from studies among community-dwelling adults 21 demonstrating strong protection against COVID-19 re-infections in individuals who have 22 previously survived an infection. [10,11] There are limited data on the influence of prior  The data from 105 nursing homes in ten states operating under the tradename Signature 8 Healthcare, predominantly from the southeast region, were reported to BASE10 Genetics, a 9 Health Management IT firm. Data were collected for any resident who had been present in a 10 participating nursing home on December 14, 2020 and included nursing home characteristics and 11 resident-level information: dates present at a facility, demographic characteristics, COVID-19 12 vaccination dates, vaccine product, and SARS-CoV-2 testing information (test date, type, and 13 result). International Classification of Diseases 10 th revision, clinical modification (ICD-10-CM) 14 codes and corresponding onset dates were extracted from nursing home electronic medical 15 records for each resident in the cohort. If two doses of vaccine were given 21 or 28 days apart, 16 missing COVID-19 vaccine product information was imputed based on the time interval between 17 dose one and dose two (i.e., 21 days for Pfizer-BioNTech and 28 days for Moderna).

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We created a retrospective closed cohort for residents who were living continuously in a 20 participating nursing home beginning on December 14, 2020 (i.e., the date the U.S. COVID-19 21 vaccination program launched in nursing homes). For each nursing home, the date facility-level vaccination began was determined as the date three or more residents were vaccinated. Residents 1 who left the facility for one or more days between December 14, 2020, and the date vaccination 2 began in their facility were excluded. Residents with a positive SARS-CoV-2 test (antigen test 3 and/or reverse transcription polymerase chain reaction (RT-PCR)) or a COVID-19 diagnosis 4 code (U07.1) on the day of or within the 90 days preceding the date vaccination began in their 5 facility were excluded. Presence of one or more underlying medical conditions, risk factors for 6 severe COVID-19, were defined based on ICD-10-codes.
[13] Presence of prior history of 7 SARS-CoV-2 infection was defined as a positive SARS-CoV-2 antigen or RT-PCR test or a 8 COVID-19 diagnosis code (U07.1) dated 91 or more days prior to the date vaccination began at a 9 resident's nursing home. 10 Resident-time follow up started on the date vaccination began at a facility until a resident 11 experienced their first positive SARS-CoV-2 test during the study follow up period or a 12 censoring event occurred. Residents were censored the first date they were not present in a 13 participating facility, on the date a third dose or a booster dose was administered, or at the end of 14 the evaluation period (November 9, 2021), whichever was earliest. All resident-days were 15 categorized as unvaccinated, pre-vaccine effect, partially vaccinated, or completed initial series 16 as defined in Table 1. Time after a completed initial vaccination series was further stratified into  We used multivariable mixed-effects Cox proportional hazards models with a time 5 dependent vaccination status to estimate hazard ratios (HRs) comparing rates of SARS-CoV-2 6 infection among residents with a completed vaccination series or partially vaccinated to rates 7 among unvaccinated residents, accounting for clustering within a nursing home using a random 8 effect and adjusting for confounders. VE was calculated as (1-HR)*100%. We evaluated the 9 effect of the following factors on VE and included them in the models: resident's age, gender,

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Analyses were conducted using SAS version 9.4 (SAS Institute).
This activity was reviewed by CDC and was conducted consistent with applicable federal law 2 and CDC policy § .

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Data for 8,301 residents who were present in a participating facility on December 14,5 2020 for at least one subsequent day were available for analysis. After exclusion criteria were  Overall, 577 (13%) residents had a positive test for SARS-CoV-2 during the study 16 period. Another 1,973 (46%) residents were censored due to leaving the participating nursing 17 home, 230 (5%) were censored at the receipt of a third dose or a booster dose, and the remaining 18 1,535 (36%) were followed through the end of the study period (November 9, 2021). At  (Table 2). Vaccine type was imputed for 34% of vaccinated residents.
During the pre-Delta period, rates of SARS-CoV-2 infections were 1.88 per 1000 resident-days 1 among unvaccinated residents and 0.20 per 1,000 resident-days among those who completed 2 their vaccination series. In the intermediate period rates were 0.09 per 1,000 resident-days for the 3 unvaccinated and 0.03 per 1,000 resident-days for the completely vaccinated. During the Delta 4 period, rates among unvaccinated were 1.40 cases per 1,000 resident-days) and among 5 completely vaccinated were 0.86 per 1,000 resident-days ( Figure 2).

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Among residents with no prior history of SARS-CoV-2 infection, the adjusted VE over 7 the entire study period for the complete vaccination series was 58% (95% CI: 44%, 69%) ( Table   8 3). Among residents with a history of SARS-CoV-2 infection, the adjusted rate of SARS-CoV-2 9 infection for residents with a complete vaccination series was similar to unvaccinated residents were 58% lower (95% CI: 26%, 76%) compared to those among unvaccinated residents 15 without a history of COVID-19. 16 During the pre-Delta period, the VE within 150 days of the receipt of the second dose 17 was 67% (95% CI: 40%, 82%) for Pfizer-BioNTech and 75% (95% CI: 32%, 91%) for Moderna 18 (   (Table 4).  [20] Lastly, a closed cohort design limited our ability to evaluate 1 effectiveness of a third dose or booster doses, as well as effectiveness during the period of 2 Omicron variant circulation (14).      within the first days 0-13 after receipt of the first dose. Partial vaccination resident-days observed 14 or more days 7 after the first dose and through 6 days after receipt of the second dose. Completed vaccination series is 7 or more 8 days after receipt of the second dose of mRNA vaccines or 14 or more days after the receipt of a dose of 9 Janssen/J&J (Table 1). Due to insufficient sample size, pre-vaccine and partial vaccination resident-days were 10 excluded from stratified analyses.