Six months after the second dose, the Pfizer Inc / BioNTech SE vaccine’s effectiveness in preventing infection by the coronavirus fell to 47% from 88%, according to data published on Monday by US health agencies when deciding whether booster shots were needed.
Ahead of peer review in August, the study’s findings were made public in the medical journal The Lancet.
The study found that even against the highly contagious Delta variant of the coronavirus, the vaccine’s effectiveness in preventing hospitalisation and death remained high at 90% for at least six months.
Researchers say the data indicates that the decline is due to waning efficacy rather than more contagious variants.
3.4 million Kaiser Permanente Southern California members were studied between December 2020 – when the vaccine first became available – and August 2021 by Pfizer and Kaiser Permanente researchers.
According to Luis Jodar, senior vice president and chief medical officer at Pfizer vaccines, “our variant-specific analysis clearly shows that the (Pfizer/BioNTech) vaccine is effective against all current variants of concern, including Delta”.
The study could have been limited by a lack of information on the study population’s adherence to masking guidelines and occupations, which could have affected the frequency of testing and the likelihood of infection.
After the first month, vaccine effectiveness against the Delta variant was 93%; this dropped to 53% after four months. Efficacy fell to 67% from 97% against other coronavirus variants.
Study leader Sara Tart of Kaiser Permanente Southern California’s Department of Research & Evaluation says, “To us, that suggests Delta is not an escape variant that completely evades vaccine protection.”
“Vaccination wouldn’t work if that were the case, so we wouldn’t have seen such high protection after vaccination. It would begin at a low level and remain there for the duration of the transaction.”
The authors cautioned that testing for variants is more likely to fail in vaccinated individuals, leading to an overestimation of the study’s variant-specific effectiveness.
A booster dose of the Pfizer/BioNTech vaccine has been approved by the U.S. Food and Drug Administration for older adults and some other Americans who are at high risk of contracting the disease. More research is needed to determine whether boosters should be recommended for everyone, according to scientists.