Two doses of the Pfizer COVID-19 vaccine also showed sustained efficacy of 93% against Delta-related hospital admission.
Two doses of the Pfizer-BioNTech (BNT162b2) vaccine protect against hospitalization for COVID-19 for at least 6 months, according to a study published this week in The Lancet.
Authors of the retrospective cohort study of more than 3 million members of Kaiser Permanente Southern California (KPSC), found the protection against hospitalization during the 6-month period was 90%.
The study also found, however, that BNT162b2 efficacy against COVID-19 infection waned significantly, from 88% (95% confidence interval [CI] 86–89) within 1 month of receiving the second dose to 47% (CI, 43-51) by the end of the 6 months.
The authors note that their findings are consistent with reports of waning immunity from the Israel Ministry of Health and from the US Centers for Disease Control and Prevention.
For the real-world assessment of BNT162b2 researchers examined 3 436 957 electronic health records from the KPSC health system from December 4, 2020-August 8, 2021. Median age of participants was 45 years and 52.4% were women. Racial and ethnic composition of the cohort was 40.5% Hispanic, 32.3% White, 11.6% Asian or Pacific Islander, and 8% were Black.
During this study period, 5.4% (184 041) were infected with COVID-19, and among these 6.6% (12 130) were hospitalized. The average time since being fully vaccinated was 3 to 4 months.
Genome sequencing and viral lineage analysis of PCR-positive samples from the study cohort showed the more infectious Delta variant was responsible for 28% of the cases within the sample. As Delta became the dominant strain in the US, the proportion of positive cases increased, escalating from 0.6% in April 2021 to 87% in July 2021.
Vaccine efficacy against infection with the Delta variant was 93% (95% CI 85–97) one month after receiving both doses but declined to 53% (95% CI 39-65) after 4 months. BNT162b2 efficacy against non-Delta variants also was high at 97% (95% CI, 95-99) 1 month after full vaccination but declined to 67% (95% CI, 45-80) at 4 months.
Effectiveness against Delta-related hospitalizations over the entire study period was high, at 93% (95% CI, 84–96) and was similar to effectiveness against hospital admissions for other (non-delta) variants.
Immunity waned for all age groups throughout the study period, suggesting that discussions about booster eligibility may need to be revisited, the authors note.
In summarizing their findings, investigators say that this variant-specific analysis “suggests that reductions in BNT162b2 effectiveness over time are likely to be primarily due to waning vaccine effectiveness rather than the delta variant escaping vaccine protection,” and point to the 90% efficacy of the vaccine against the delta variant within 1 month of completing the second dose of the vaccine.
Added support for this conclusion, they say, are “reductions in effectiveness in infections by time since being fully vaccinated were observed irrespective of SARS-CoV-2 variant, and effectiveness against hospital admissions due to the delta variant was very high over the entire study period.”
Reference: Tartof SY, Slezak JM, Fischer H, et al. Effectiveness of mRNA BNT162b2 COVID-19 vaccine up to 6 months in a large integrated health system in the USA: a retrospective cohort study. Published online October 04, 2021, DOI:https://doi.org/10.1016/S0140-6736(21)02183-8.