Using nationwide data to examine the relative effectiveness of Covid-19 vaccination against omicron amongst a population with low prior rates of SARS-CoV-2 infection: A cohort study

Background: We used nationwide data to examine relative vaccine effectiveness (rVE) of the Comirnaty mRNA vaccine (Pfizer–BioNTech (original), hereafter Comirnaty, against the Omicron variants (BA.1 and BA.2) during 2022 in Aotearoa New Zealand (NZ). 

Method: We analysed a national cohort of 3.15 million adults (18+ years) who had received at least two doses of Comirnaty by March 2022. Data sources included national administrative records of vaccination, hospitalisation, ICU admission and death. Cox regression was used to estimate hazard ratios in recipients of three vaccine doses compared to recipients of two doses. 

Results: Amongst adults, three vaccine doses provided significantly greater protection against hospitalisation attributable to Covid-19 than two doses, relative vaccine effectiveness (rVE) was 50 %, 95 % Confidence Interval (CI) 45–55 %) at peak virus circulation. The vaccine was effective for Māori, Pacific Peoples and those aged over 50 years however, the protection given by vaccination waned throughout the study period. The booster was also significantly more effective at preventing ICU admission or death with an rVE (3 vs 2 doses) of 53 %, 95 % CI 49–55 %. It was consistent for Māori, 49 %, 95 %CI 41–56 % and Pacific Peoples 52 %, 95 % CI 41–62 %, and those aged over 50 years, 54 %, 95 % CI 51–57 %. 

Conclusion: The study provides important insights into relative vaccine effectiveness of the Comirnaty booster doses against Omicron variants in NZ in 2022 in an infection naive population. The findings highlight the importance of booster doses in combatting hospitalisation, ICU admission and death during the 2022 Omicron wave.