Effectiveness of a COVID-19 Additional Primary or Booster Vaccine Dose in Preventing SARS-CoV-2 Infection Among Nursing Home Residents During Widespread Circulation of the Omicron Variant – United States, February 14-March 27, 2022

Nursing home residents have experienced disproportionally high levels of COVID-19–associated morbidity and mortality and were prioritized for early COVID-19 vaccination (1). Following reported declines in vaccine-induced immunity after primary series vaccination, defined as receipt of 2 primary doses of an mRNA vaccine (BNT162b2 [Pfizer-BioNTech] or mRNA-1273 [Moderna]) or 1 primary dose of Ad26.COV2 (Johnson & Johnson [Janssen]) vaccine (2), CDC recommended that all persons aged ≥12 years receive a COVID-19 booster vaccine dose.* Moderately to severely immunocompromised persons, a group that includes many nursing home residents, are also recommended to receive an additional primary COVID-19 vaccine dose.^†

Data on vaccine effectiveness (VE) of an additional primary or booster dose against infection with SARS-CoV-2 (the virus that causes COVID-19) among nursing home residents are limited, especially against the highly transmissible B.1.1.529 and BA.2 (Omicron) variants. Weekly COVID-19 surveillance and vaccination coverage data among nursing home residents, reported by skilled nursing facilities (SNFs) to CDC’s National Healthcare Safety Network (NHSN)^§ during February 14–March 27, 2022, when the Omicron variant accounted for >99% of sequenced isolates, were analyzed to estimate relative VE against infection for any COVID-19 additional primary or booster dose compared with primary series vaccination.

After adjusting for calendar week and variability across SNFs, relative VE of a COVID-19 additional primary or booster dose was 46.9% (95% CI = 44.8%–48.9%). These findings indicate that among nursing home residents, COVID-19 additional primary or booster doses provide greater protection against Omicron variant infection than does primary series vaccination alone.

All immunocompromised nursing home residents should receive an additional primary dose, and all nursing home residents should receive a booster dose, when eligible, to protect against COVID-19. Efforts to keep nursing home residents up to date with vaccination should be implemented in conjunction with other COVID-19 prevention strategies, including testing and vaccination of nursing home staff members and visitors.

Each week, nursing homes certified by Centers for Medicaid & Medicare Services (CMS) report incident confirmed SARS-CoV-2 infections among residents and staff members, by vaccination status, to NHSN. This study was limited to case data reported by CMS-certified SNFs, which account for >90% of nursing homes reporting COVID-19 data to NHSN, during February 14–March 27, 2022, when the Omicron variant accounted for >99% of sequenced isolates nationwide.^¶ COVID-19 case ascertainment at CMS-certified SNFs during the study period was high, because of guidelines recommending weekly testing of all residents and staff members if a single SARS-CoV-2 infection was identified in a facility.**

At SNFs with contact tracing capacity, only close contacts of an infected resident or staff member were tested. Vaccination status of infected persons was categorized as 1) vaccinated with a primary series only (receipt of 2 doses of an mRNA vaccine or 1 dose of the Janssen COVID-19 vaccine ≥14 days before a SARS-CoV-2–positive test result, or receipt of an additional primary or booster dose <14 days before a SARS-CoV-2–positive test result), 2) vaccinated with an additional or booster dose (receipt of any authorized COVID-19 additional primary or booster dose ≥14 days before a SARS-CoV-2–positive test result),^†† 3) unvaccinated (no COVID-19 vaccine dose or a single dose <14 days before a SARS-CoV-2–positive test result), and 4) other (receipt of a single mRNA vaccine dose ≥14 days before a SARS-CoV-2–positive test result or unspecified vaccination).

SNFs also report the weekly census of residents by vaccination status. Resident-weeks were calculated as the aggregate of weekly resident counts, by vaccination status, at each SNF. In this study, weekly case counts by vaccination status in each SNF were paired with weekly resident counts by vaccination status from 2 weeks earlier.

Data from SNFs that reported no additional or booster dose coverage throughout the study period were excluded.^§§ In addition, weekly case count reports were excluded if a facility did not report corresponding resident counts for the preceding 2 weeks. Crude infection rates by vaccination status were calculated with 95% CIs based on the binominal distribution.

Infection rates among residents who received an additional or booster dose were compared with those who received primary series vaccination only to estimate relative additional or booster dose VE.

The effectiveness of primary series vaccination or an additional or booster dose compared with no vaccination (i.e., absolute VEs) were not reported because of differences in visitation, quarantine, and masking policies between unvaccinated residents and vaccinated residents based on updated CMS guidelines,^¶¶ as well as the inability to adjust for confounding because of these factors. Product-specific VE also could not be estimated because SNFs only reported relevant vaccine product information in weekly resident count reports and not within weekly case count reports.

A zero-inflated Poisson mixed effects model, which adjusted for calendar week using quadratic splines and included SNF as a random effect to account for variability across facilities, was used to estimate the ratio of infection rates between residents who received an additional or booster dose and those who received primary series vaccination only. Relative VE was estimated as 1 minus the rate ratio multiplied by 100.

The following characteristics were evaluated as potential confounders of relative VE: 1) weekly cumulative staff member and resident SARS-CoV-2 infection rates at each SNF during the study period (since May 8, 2020), 2) weekly SNF-level staff member COVID-19 vaccination coverage, 3) each SNF’s county-level incidence of SARS-CoV-2 infection, and 4) each SNF county’s CDC Social Vulnerability Index score.*** A 10% change-in-estimate criterion for the regression coefficient was used to evaluate covariates; none met this criterion and thus none were included in the final model. Data analysis was conducted using SAS software (version 9.4; SAS Institute) and R software (version 4.1.2; R Foundation). This activity was reviewed by CDC and was conducted consistent with federal laws and institutional policies.^†††

Overall, 15,090 SNFs provided 89,671 weekly case count reports during February 14–March 27, 2022, and 15,102 SNFs provided 89,969 weekly resident count reports during January 31–March 13, 2022. After applying exclusion criteria and pairing SNF-level weekly case with corresponding resident data, the analysis included 85,494 reports from 14,758 SNFs.

The median weekly number of residents reported was 1,126,198 (IQR = 1,124,328–1,126,709); approximately 22% of whom had received primary series vaccination only, and 65% of whom had received an additional or booster dose. Among residents who had received primary series vaccination or an additional or booster dose, >90% had received mRNA COVID-19 vaccines.

Crude weekly confirmed SARS-CoV-2 infection rates declined across all vaccination groups during the study period (Figure); however, rates of infection among residents with an additional or booster dose were consistently lower than those among residents with primary series vaccination only or among unvaccinated residents.

Overall, 7,510 cases were confirmed among 1,509,674 resident-weeks with primary series vaccination only and 11,334 cases were confirmed among 4,416,401 resident-weeks with an additional or booster dose (Table). The adjusted relative VE against infection for an additional or booster dose versus primary series vaccination only was 46.9%.