Pfizer/BioNTech COVID-19 vaccine efficacy wanes after six months, study shows

The data, which was published in the Lancet medical journal, had been previously released in August ahead of peer review.

The analysis showed that

the vaccine's effectiveness in preventing hospitalization and death remained high at 90% for at least six months, even against the highly contagious Delta variant of the coronavirus.

Our results provide support for high effectiveness of BNT162b2 [Pfizer/BioNTech's COVID-19 vaccine] against hospital admissions up until around 6 months after being fully vaccinated, even in the face of widespread dissemination of the delta variant. Reduction in vaccine effectiveness against SARS-CoV-2 infections over time is probably primarily due to waning immunity with time rather than the delta variant escaping vaccine protection

, researchers said.

The study, conducted between 14 December, 2020, and 8 August, 2021, included nearly 3.44 million people, with a median age of 45 years, of whom 52.4% were female and 47.6% were male.

Key findings

• For fully vaccinated individuals, effectiveness against SARS-CoV-2 infections was 73% (95% CI 72–74) and against COVID-19-related hospital admissions was 90% (89–92).
• Effectiveness against infections declined from 88% (95% CI 86–89) during the first month after full vaccination to 47% (43–51) after 5 months.
• Among sequenced infections, vaccine effectiveness against infections of the delta variant was high during the first month after full vaccination (93% [95% CI 85–97]) but declined to 53% [39–65] after 4 months.
• Effectiveness against other (non-delta) variants the first month after full vaccination was also high at 97% (95% CI 95–99), but waned to 67% (45–80) at 4–5 months.
• Vaccine effectiveness against hospital admissions for infections with the delta variant for all ages was high overall, (93% [95% CI 84–96]) up to 6 months.

"Our variant-specific analysis clearly shows that the (Pfizer/BioNTech) vaccine is effective against all current variants of concern, including Delta," said Luis Jodar, senior vice president and chief medical officer at Pfizer vaccines.

A potential limitation of the study was a lack of data on adherence to masking guidelines, social interactions, and occupations in the study population, which could have affected frequency of testing and likelihood of exposure to the virus.

"To us, that suggests Delta is not an escape variant that is completely evading vaccine protection," said study leader Sara Tartof with Kaiser Permanente Southern California's Department of Research & Evaluation.

"If it was, we would probably not have seen high protection after vaccination, because vaccination would not be working in that case. It would start low, and stay low."

The researchers said effectiveness was lowest for PCR-positive specimens for which a sequence could not be determined.

"These specimens had higher Ct values than other PCR-positive specimens, which probably corresponded to milder or asymptomatic infections. Thus, our vaccine effectiveness estimates against SARS-CoV-2 infections and hospital admissions could be muted by mild or asymptomatic infections and are not directly comparable to estimates of effectiveness against symptomatic disease. Sequencing was more likely to fail in samples from vaccinated individuals due to lower viral loads, which could lead to an overestimate of variant-specific effectiveness."

The U.S. Food and Drug Administration has authorized the use of a booster dose of the Pfizer/BioNTech vaccine for older adults and some Americans at high-risk of getting infected. Scientists have called for more data on whether boosters should be recommended for all.

Cover photo: Getty Images