Scientists don’t know how long protective effects of the vaccines will last, but clinical trials, which are continuing, are trying to determine the answer.
Dr. Peter Marks, the head of the FDA’s Center for Biologics Evaluation and Research, said in December that based on the experience of people who were immunized earliest in the trials, it appears that protection against the disease lasts at least several months.
By the summer of 2021, a few studies have shown a drop in vaccine effectiveness against infection, but they continue to show high effectiveness against severe disease. For example, one study, published in the CDC’s Morbidity and Mortality Weekly Report, looked at adults living in New York state, the vast majority of whom had been immunized with mRNA vaccines. It found vaccine effectiveness against any kind of lab-confirmed infection, whether symptomatic or not, declined from 92% in early May to 75% in late July. But it found that vaccine protection against hospitalization remained at 90% or higher for each week analyzed.
An MMWR study of a more vulnerable population — U.S. nursing home residents — found that the effectiveness of the mRNA vaccines against lab-confirmed infections fell from 75%, before the delta variant predominated in the U.S., to 53% with delta.
Experts with the FDA and CDC are evaluating whether to recommend a third dose of the mRNA vaccines, or a “booster” shot. Many scientists say it’s not yet clear from the available evidence that boosters are needed. For more, see “Biden’s Controversial COVID-19 Vaccine Booster Plan.”
On Sept. 17, the FDA’s Vaccines and Related Biological Products Advisory Committee voted against recommending a booster shot of the Pfizer/BioNTech vaccine for everyone 16 and older. But the committee unanimously recommended an emergency use authorization for a booster dose given at least six months after completion of the primary two-dose series for those age 65 and older and those at “high risk of severe COVID-19.”
The nation’s health agencies had already signed off on providing a third dose of the mRNA vaccines to immunocompromised people. The FDA authorized additional doses in that population for emergency use on Aug. 12, and the CDC’s Advisory Committee on Immunization Practices and the agency endorsed that view the following day.