Federal health officials are expected Wednesday to present evidence for why people are likely to need Covid-19 boosters eight months after their second doses of a vaccine, according to sources with the Centers for Disease Control and Prevention.
The eight-month time frame is most likely based on findings from both the U.S. and abroad looking at how the vaccines have held up over time — and whether they can stand up to the hypertransmissible delta variant of the coronavirus that has overtaken the country.
“Delta is forcing this discussion” on boosters, said Dr. Colleen Kraft, associate chief medical officer at Emory University Hospital in Atlanta.
The variant now accounts for nearly 100 percent of new Covid-19 cases in the U.S., according to the CDC. The seven-day average of Covid-19 cases has soared by 700 percent since the beginning of July, when delta became the dominant strain in the U.S.
Questions remain about how well the vaccines fare against the delta variant, as well as whether protection simply wanes over time. Indeed, experts said they are eager to see any new data that led to the government’s proposed timeline for booster shots.
Data from countries that began their vaccination campaigns before the U.S. provide important clues to when boosters might be needed here.
Limited research from Israel, one of the first countries to begin widespread vaccination, which has almost exclusively used the vaccine from Pfizer-BioNTech, is likely to have played a role in the administration's expected rollout of timing for booster doses, doctors said.
“The data are strongly suggestive that people who got vaccinated early in Israel — that is, in January — are seeing infections in the vaccinated at a higher rate than people who got vaccinated in April,” said Dr. Sankar Swaminathan, chief of the infectious diseases division at University of Utah Health.
Data posted on the Israeli Health Ministry’s website indicate that the vaccines administered to people over age 65 in January now offer just 55 percent protection against severe illness.
That’s a concern, particularly with how contagious the delta variant is, said an expert in pulmonary and critical care, Dr. Benjamin Singer, an assistant professor of medicine at the Northwestern University Feinberg School of Medicine.
The variant has been associated with breakthrough infections, and while such infections are rare and generally much less severe, studies indicate that fully vaccinated people can spread the virus if infected.
As for why the booster shots would be recommended at eight months, Singer said, “it’s not entirely clear, but it’s probably a combination of waning immunity over time and the fact the current circulating variants are just that much more contagious and spread that much more easily.”
Still, the data from Israel come with caveats. Swaminathan said those who got vaccinated the earliest there were in high-risk populations, who may already carry a higher chance of getting breakthrough infections, making it unclear whether their risk of infection or hospitalization eight months out would match that of the general population.
Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine and co-director of the Texas Children’s Hospital Center for Vaccine Development, said, “All we’re getting is snippets of data on the Israel Ministry of Health website.” He said he was curious whether, in addition to a decline in efficacy against mild infections, there was a significant drop in protection against hospitalizations for vaccinated people over time.
John Grabenstein, a former executive director of medical affairs for vaccines at Merck, as well as a former immunologist for the Defense Department, agreed that more data need to be presented.
"The Israeli data is a snapshot, and I don't want to make a quick decision based on one snapshot,” Grabenstein said. "I want to see multiple pictures, multiple views of the same thing, with the numbers beginning to coalesce around the same conclusion.”
A recent study from the Mayo Clinic, however, also suggested waning immunity from the vaccines. The research, which hasn’t yet been published in a peer-reviewed journal, found that protection against infection during the height of the delta variant’s spread fell to 76 percent among those who had received the Moderna shots and 42 percent among those who got the Pfizer shots.
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“There seems to be a dip in terms of immune responses,” said an author of the paper, Dr. Abinash Virk, an infectious disease specialist at the Mayo Clinic. Her team is trying to determine whether the dip is truly due to waning immunity or whether the delta variant is mostly to blame.
“It’s really difficult to sort this all out, because this is all happening concurrently,” Virk said, adding that mask mandates also went away as the delta variant began spreading.
Some doctors questioned why the U.S. guidance appeared to be a blanket recommendation for everyone.
“What’s surprising about it is that it may be a universal recommendation that is based on time rather than on risk factor, and that would seem to me to be a bit unusual, especially when we look at the rest of the planet, who have, in some areas, very low access to vaccines,” said Dr. Buddy Creech, director of the Vanderbilt Vaccine Research Program at the Vanderbilt University Medical Center.
It’s also not clear what effect booster shots could have on the trajectory of the pandemic.
Some predicted that boosters could reduce serious infections among immunocompromised people or others at high risk for complications. But to make a real dent in reducing transmission — the key to ending the pandemic — those who haven’t gotten vaccinated at all will have to get their first doses, Singer said.
“The boosters probably have a limited impact on reducing the worst impacts, because it’s really unvaccinated people who are making up the supermajority of hospitalizations, ICU admissions and deaths,” Singer said. “If you have less infection, including breakthrough infections, you might break that train of transmission, but if you’re looking now, most of the transmission is occurring among people who have had zero vaccines.”