TABLE. Relative effectiveness of additional COVID-19 primary or booster vaccine doses in preventing SARS-CoV-2 infection among residents of skilled nursing facilities compared with primary series vaccination only — National Healthcare Safety Network, United States, February 14–March 27, 2022.

Vaccination status*	No. of resident-weeks†	No. of cases§	Crude infection rate¶ (95% CI)	Vaccine effectiveness % (95% CI)
				Unadjusted**	Adjusted††
Primary series	1,509,674	7,510	5.0 (4.9–5.1)	Ref	Ref
Additional or booster dose	4,416,401	11,334	2.6 (2.5–2.7)	49.3 (47.3–51.3)	46.9 (44.8–48.9)

Abbreviations: Ref = referent group; SNF = skilled nursing facility.

* Residents who completed a primary vaccination series were those who received 2 primary doses of an mRNA vaccine Pfizer-BioNTech or Moderna or 1 primary dose of Janssen vaccine. Residents with additional or booster dose vaccination were those who received an additional primary vaccine dose ≥28 days after the initial primary series or a booster dose ≥5 months after completion of an mRNA primary series or ≥2 months after 1 primary Janssen vaccine dose. Residents with an additional or booster dose included those who received additional primary vaccine doses and a booster dose.

^† Resident-weeks were calculated as the aggregate of weekly resident counts, by vaccination status, at each SNF.

^§ Cases among residents with primary series vaccination were defined as infections in residents who had received primary series vaccination ≥14 days before a SARS-CoV-2–positive test result or received an additional or booster dose <14 days before a positive SARS-CoV-2 test result. Cases among residents with additional or booster dose vaccination were defined as infections in residents who had received an additional primary or booster dose ≥14 days before a positive SARS-CoV-2 test result. For analysis, weekly case counts by vaccination status in each SNF were paired with weekly resident counts by vaccination status from 2 weeks earlier (January 31–March 13, 2022).

^¶ Infections per 1,000 resident-weeks.

** Results from a zero-inflated Poisson mixed effects model with random effects for SNF. Vaccine effectiveness was estimated as 1 minus the rate ratio multiplied by 100, with the rate ratio comparing infection rates among residents vaccinated with an additional or booster dose to residents with primary series vaccination only.

^†† Results from the same model while also controlling for calendar week using quadratic splines.