As families and friends gather for the holidays and concern for a winter surge grows, it’s no coincidence that public health officials and federal and local leaders made the big push last week to expand COVID-19 vaccine boosters for all adults
But as more people get additional shots, a big question lurks in the background: How long will those boosters last?
Anthony Fauci, the nation's top infectious disease expert, said Sunday on ABC that the hope is boosters will offer long-lasting protection so additional doses aren’t needed.
“We would hope, and this is something that we're looking at very carefully, that that third shot with the mRNA not only boosts you way up, but increases the durability so that you will not necessarily need it every six months or a year,” he said, referring to the Pfizer and Moderna vaccines, which use the same messenger RNA platform. “We're hoping it pushes it out more.”
But right now, the longevity of booster doses is unknown, and finding out will take some time.
“We know very little about how long boosters last,” Julie Parsonnet, a Stanford expert in epidemiology and population health, told The Chronicle in an email. “The hope is that further vaccinations will be unnecessary, but we don’t have any evidence yet to support that hope.”
The U.S. is still early in its boosting campaign, “so we need time to be able to tell how long-lived immunity will be after the third dose,” said Nadia Roan, an investigator at the Gladstone Institutes in San Francisco who studies vaccine-induced immunology, in an email.
In mid-August, the FDA granted emergency use authorization for a third vaccine dose for immunocompromised individuals, and in late September, did the same for elderly people and others at high-risk for severe infection.
Roan said there’s a chance that all we need is three doses, but “it's also possible that repeated COVID vaccine boosting could become a regular thing,” like the annual flu vaccine.
Parsonnet said another concern is whether coronavirus variants arise that escape vaccine immunity.
“Given what we know about coronaviruses more generally, however, I suspect we may need occasional revaccination,” she said.
What we know is that boosted individuals are “more protected from overall breakthrough infection than non-boosted vaccinated individuals,” Roan said.
“Right now, we know that vaccinating at six months is safe and effective,” she said. “We also know that the vaccines wane a bit over this time period. That is the reason this timing was chosen.”
Israel’s booster rollout offers lessons, showing that third shots were effective early on, according to Parsonnet. The country, which uses the Pfizer vaccine, approved boosters for older people in late July, and began offering them to everyone 16 and older at the end of August.
“Given the date of onset of third dosing, we’ll see six-month efficacy rates sometime in the late winter,” she said, adding that information about waning antibody levels might come sooner. “Those efficacy rates will start to inform whether further shots will be needed.”
She said she doesn’t expect any helpful information about Moderna and Johnson & Johnson boosters, which rolled out later, until next summer. But if the data looks promising, researchers will then examine numbers at the nine-month and one-year marks.
Another big question is the dosing intervals, which Parsonnet said is an even tougher challenge.
“In the past, there was no rush to vaccinate everyone, so we had the luxury of trying out multiple strategies,” she said. “But with COVID, we’ve needed to push forward quickly. It may take years to know what the optimal dosing intervals are.”
She said we’ll first find out the right dosing of the third shot, since people will get the shots at different intervals: Some may get them right at six months after their second dose, and others will wait longer.
Roan said she doesn’t think waiting six months or eight months should make a significant difference, because the memory immunity cells (called B and T cells) have already been formed.
“What the third dose will do is to reawaken those memory cells, and get them to expand in numbers, and in the case of B cells, start producing antibodies again,” she explained. “The elevation in antibody levels after the boosting provides a robust first line of defense against breakthrough infection.”
The antibodies won’t stay at high levels forever, but the memory immune cells will be more long-lasting, which is “important in providing protection against severe COVID-19,” she said. This is why despite waning immunity after the second vaccine dose, the vaccine continues to provide strong protection against hospitalizations and death.
Roan said boosters are likely most beneficial to immunocompromised individuals who didn’t mount an antibody response to the first two doses of mRNA vaccine, and were able to elicit some antibodies after a third dose.
However, Parsonnet said, some immunocompromised people may never respond to vaccines no matter how many doses they receive, citing reports that have shown only half of immunocompromised individuals have responded to third doses. So it’s important that all adults get an additional vaccine dose in order to protect everyone, said Parsonnet.
“We are seeing enough breakthrough infections — a small proportion of which are serious — in the general population that boosting everyone will both prevent serious illness in the vaccinated and, perhaps even more importantly, limit transmission to those who don’t respond effectively to vaccines.”
Kellie Hwang is a San Francisco Chronicle staff writer. Email: kellie.hwang@sfchronicle.com Twitter: @KellieHwang
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