Coronavirus Disease 19 (COVID-19) Vaccine Effectiveness Against Symptomatic Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Infection During Delta-Dominant and Omicron-Dominant Periods in Japan: A Multicenter Prospective Case-control Study (Factors Associated with SARS-CoV-2 Infection and the Effectiveness of COVID-19 Vaccines Study)

Abstract

Background Although several COVID-19 vaccines initially showed high efficacy, there have been concerns due to waning immunity and the emergence of variants with immune escape capacity. Methods A test-negative design case-control study was conducted in 16 healthcare facilities in Japan during the Delta-dominant period (August-September 2021) and the Omicron-dominant period (January-March 2022). Vaccine effectiveness (VE) against symptomatic SARS-CoV-2 infection was calculated for 2 doses for the Delta-dominant period and 2 or 3 doses for the Omicron-dominant period, compared to unvaccinated individuals. Results The analysis included 5795 individuals with 2595 (44.8%) cases. Among vaccinees, 2242 (55.8%) received BNT162b2 and 1624 (40.4%) received mRNA-1273 at manufacturer-recommended intervals. During the Delta-dominant period, VE was 88% (95% CI: 82-93) 14 days-3 months after dose 2 and 87% (95% CI: 38-97) 3-6 months after dose 2. During the Omicron-dominant period, VE was 56% (95% CI: 37-70) 14 days-3 months since dose 2, 52% (95% CI: 40-62) 3-6 months after dose 2, 49% (95% CI: 34-61) 6 + months after dose 2, and 74% (95% CI: 62-83) 14 + days after dose 3. Restricting to individuals at high risk of severe COVID-19 and additional adjustment for preventive measures (i.e. mask-wearing/high-risk behaviors) yielded similar estimates, respectively. Conclusions In Japan where most are infection-naïve and strict prevention measures are maintained regardless of vaccination status, 2-dose mRNA vaccines provided high protection against symptomatic infection during the Delta-dominant period and moderate protection during the Omicron-dominant period. Among individuals who received an mRNA booster dose, VE recovered to a high level.

Estimation of mRNA COVID-19 Vaccination Effectiveness in Tokyo for Omicron Variants BA.2 and BA.5 -Effect of Social Behavior-

“…We compared with the result in cohort study in Tokyo and suburb area [31] suggested individual VE for Delta and Omicron-dominant period was 88% (95% CI: 82–93) and 56% (95% CI: 37–70) for 14 days to 3 months after second dose to confirm our numerical derivation. Individual VE averaged over 14 days to 3 months were 88% and 53%, which are in excellent agreement with our computational model.…”

“…We compared with the result in cohort study in Tokyo and suburb area [31] suggested individual VE for Delta and Omicron-dominant period was 88% (95% CI: 82-93) and 56% (95% CI: 37-70) for 14 days to 3 months after second dose to confirm our numerical derivation.…”

National seroepidemiological study of COVID‐19 after the initial rollout of vaccines: Before and at the peak of the Omicron‐dominant period in Japan

“…Quantitative assessment of antibodies revealed that, although there was a decreasing trend in antibody titers as the age increased, and individuals with underlying diseases tended to have lower anti‐S antibodies partly owing to the rollout timing, individuals who received three doses of vaccines had higher titers than individuals who received two doses, regardless of the age group. Although correlates of protection for infection, symptom development, and severe disease are not well understood, 21 , 22 , 23 these results were reassuring and, together with published epidemiological evidence, 24 , 25 , 26 support a continued rollout of a third booster dose.…”

Real-world vaccine effectiveness of mRNA vaccines for SARS-CoV-2; a test-negative case-control study in a medium-sized clinic

“…A sample size of 216 (e.g., 22 cases and 194 controls) was required to detect a possible 75% VE for preventing the onset of COVID-19 symptoms during the period of Delta variant predominance. We also assumed a VE of 50% against the Omicron variant, based on the report by the National Institute of Infectious Diseases, 22 a test-positive rate of 50% observed in our facility during the previous month, and a vaccination rate of 80% as observed in Kawasaki City during that period. A sample size of 343 (e.g., 172 cases and 172 controls) was required to detect a possible VE of 50% for preventing the onset of COVID-19 symptoms during the period of Omicron variant surge.…”