- Researchers say the effectiveness of COVID-19 vaccines does decrease over time, although it’s not certain yet if age is a factor.
- They say this decline makes getting a booster shot all the more important, especially with the spread of the Delta variant.
- Annual boosters are common for vaccines, including measles, flu, and tetanus.
Vaccinations against COVID-19 remain highly effective against serious illness caused by the coronavirus.
However, some analyses have shown that the protection offered by these vaccines may decrease over time.
That’s why booster shots are being recommended beginning this month for people who were among the first to get vaccinated.
Unlike the initial round of COVID-19 vaccinations, however, age may not determine who will get the boosters first.
Clinical studies have shown that the mRNA vaccines developed by Pfizer-BioNTech and Moderna were initially more than 90 percent effective against serious COVID-19 disease.
The Johnson & Johnson vaccine was found to be somewhat less effective, but still highly protective against hospitalization.
However, the University of California San Diego researchers wrote in the New England Journal of Medicine last week that mRNA vaccine effectiveness against symptomatic COVID-19 cases had dipped to 65 percent by July 2021, with the drop-off attributed to waning effectiveness over time.
“The decline in effectiveness is not entirely surprising,” said Dr. Francesca Torriani, professor of clinical medicine in the division of infectious diseases and global public health at the UC San Diego School of Medicine, as well as a program director of infection prevention and clinical epidemiology at UC San Diego Health.
“Clinical trial data suggested decreased effectiveness would occur several months after full vaccination, but our findings indicate that confronted by the Delta variant, vaccine effectiveness for mildly symptomatic disease was considerably lower and waned 6 to 8 months after completing vaccination,” Torriani explained.
Jonathan H. Watanabe, PharmD, PhD, a COVID-19 health outcomes researcher and associate dean of assessment and quality at the University of California Irvine, told Healthline that research shows that the effectiveness of mRNA vaccines seems to decline about 6 percent every 2 months.
Meanwhile, a study from Israel concluded that a third booster shot of the mRNA vaccines appeared to significantly restore protective effects.
“We found that 7–13 days after the booster shot there is a 48–68 percent reduction in the odds of testing positive for SARS-CoV-2 infection, and that 14–20 days after the booster the marginal effectiveness increases to 70–84 percent,” the researchers from Maccabi Healthcare Services reported.
“There’s not a whole lot to argue about in terms of… administering an antibody boost against the most prevalent [Delta] variant,” said Watanabe.
In mid-August, the U.S. Department of Health and Human Services (HHS) announced plans to offer COVID-19 booster shots to people who had previously received both doses of the Pfizer or Moderna vaccines.
Officials cited concerns that vaccine effectiveness “could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout.”
“The available data make very clear that protection against SARS-CoV-2 infection begins to decrease over time following the initial doses of vaccination, and in association with the dominance of the Delta variant, we are starting to see evidence of reduced protection against mild and moderate disease,” according to the policy statement issued by HHS.
HHS officials said the booster shots would be available starting the week of September 20.
Priority will be given to those who received their initial doses of the mRNA vaccines 8 months earlier, “including many healthcare providers, nursing home residents, and other seniors,” as well as residents of long-term care facilities, according to HHS.
Unlike the first mRNA rollout in early 2021, age won’t directly determine priority for booster shots.
However, Americans who got their initial vaccination as soon as it was available for those ages 65 and older would still be among the first to be eligible for the booster shots.
Studies on vaccine waning have been inconclusive about whether or not protection weakens more rapidly among people over age 65.
The study from Israel suggested that effectiveness among older adults declined more than among other age groups. But a
Booster shots for recipients of the Johnson & Johnson vaccines also “will likely be needed,” according to HHS, although the time frame will differ because administration of that vaccine didn’t begin until March 2021.
Further booster shots may also be required as long as larger portions of the population remain unvaccinated and new variants continue to develop through uncontrolled community spread of the disease, said Watanabe.
“The reality of this vaccine is that until we really get to that herd immunity of 70 percent or higher, we’re going to keep doing this,” he said.
Watanabe cautioned against concluding that COVID-19 vaccines don’t work simply because booster shots are being recommended.
Flu shots are needed every year, he noted, and booster shots are common for many other commonly administered vaccines, including tetanus, hepatitis, and measles.
“Although efficacy may have waned, the vaccines still provide highly effective protection against severe COVID-19 disease,” he said.
