Updated Covid-19 boosters that carry instructions to arm the body against currently circulating Omicron subvariants offer some protection against infections, according to the first study to look at how the boosters are performing in the real world. However, the protection is not as high as that provided by the original vaccine against earlier coronavirus variants, the researchers say.
Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, called the new data "really quite good."
"Please, for your own safety, for that of your family, get your updated Covid-19 shot as soon as you're eligible to protect yourself, your family and your community," Fauci said at a White House briefing Tuesday.
Uptake of the bivalent boosters, which protect against the BA.4/5 subvariants as well as the original virus strain, has been remarkably slow. Only 11% of eligible Americans have gotten them since they became available in early September.
The new study found that the updated boosters work about like the original boosters. They protect against symptomatic infection in the range of 40% to 60%, meaning that even when vaccine protection is its most potent, about a month after getting the shot, people may still be vulnerable to breakthrough infections.
That's in about the same range as typical efficacy for flu vaccines. Over the past 10 years, CDC data shows, the effectiveness of the seasonal flu vaccines has ranged from a low of 19% to a high of around 52% against needing to see a doctor because of the flu. The effectiveness varies depending on how similar the strains in the vaccine are to the strains that end up making people sick.
The authors of the new study say people should realize that the Covid-19 vaccines are no longer more than 90% protective against symptomatic infections, as they were when they were first introduced in 2020.
"Unfortunately, the 90% to 100% protection was what we saw during like pre-Delta time. And so with Delta, we saw it drop into the 70% range, and then for Omicron, we saw it drop even lower, to the 50% range. And so I think what we're seeing here is that the bivalent vaccine really brings you back to that sort of effectiveness that we would have seen immediately after past boosters, which is great. That's where we want it to get," said Dr. Ruth Link-Gelles, an epidemiologist at the US Centers for Disease Control and Prevention.
Vaccines are only one tool to stay well
"This protection is not 100%, but it is something," Link-Gelles said. "Especially going into the holidays where you're likely to be traveling, spending time with elderly relatives, with vulnerable people. I think having some protection from infection and therefore some protection from infecting your loved one is better than having no protection at all."
Link-Gelles says it also means that people should continue to adopt a layered approach to protection, utilizing rapid tests, good-quality masks and ventilation as a comprehensive approach, rather than relying on vaccines alone.
"This should be sort of one of the things in your toolbox for protecting yourself and your family," she said. "Personally, we're my family is all vaccinated up to date, but I think if we go to the airport tomorrow, we'll be wearing our N95 [masks] because we're seeing elderly relatives this weekend. And while we of course trust the vaccines, and I'm not super worried about a mild infection in myself or my healthy husband, we certainly would not want to infect his grandmother."
Link-Gelles added that she expects that vaccine protection against severe outcomes from Covid-19, like hospitalization and death, will be higher, but that data isn't in yet.
The study, which was led by CDC scientists, relied on health records from more than 360,000 tests given at nearly 10,000 retail pharmacies between Sept. 14 and Nov. 11, a period when the BA.4 and BA.5 subvariants were causing most Covid-19 infections in the US. The study included people ages 18 and up who had Covid-19 symptoms and were not immunocompromised.
The study looked at how effective the boosters were in two ways: Researchers calculated a value called absolute vaccine effectiveness, which compared the odds of symptomatic infection in people who received bivalent boosters with those who reported being unvaccinated. They also calculated relative vaccine effectiveness, which looked at the odds of symptomatic infection in people who received updated bivalent boosters compared with those who had two, three or four doses of the original single-strain vaccine.
Compared with people who were unvaccinated, adults 18 to 49 who had gotten bivalent boosters were 43% less likely to get sick with a Covid-19 infection. Older adults, who tend to have weaker immune function, got less protection. Those ages 50 to 64 were 28% less likely, and those ages 65 and up were 22% less likely to get sick with Covid-19 than the unvaccinated group.
The relative vaccine effectiveness showed the added protection people might expect on top of whatever protection they had left after previous vaccine doses. If a person was two to three months past their last vaccine dose, the bivalent boosters added an average of 30% protection for those who were ages 18 to 49, 31% more protection if they were 50 to 64, and 28% more protection if they were 65 or older. At 3 months after their last booster, people ages 50 and older still had about 20% protection from Covid-19 illness, CDC data show. So overall, the updated boosters got them to around 50% effectiveness against symptomatic infection.
If a person was more than eight months away from their last vaccine dose, they got more protection from the boosters. But Link-Gelles said that by eight months, there was little protection left from previous shots against Omicron and its variants, meaning the vaccine effectiveness for this group was probably close to their overall protection against infection.
Those ages 18 to 49 who were eight months or more past their last dose of a vaccine had 56% added protection against a Covid-19 infection with symptoms; adults 50 to 64 had 48% added protection, and adults over 65 had 43% added protection, on top of whatever was left from previous vaccinations.
Short, modest protection from boosters
John Moore, an immunologist and microbiologist at Weill Cornell Medicine, said it boils down to the fact that that boosters will probably cut your risk of getting sick by about 50%, and that protection probably won't last.
"Having a booster will give you some additional protection against infection for a short term, which is always what we see with a booster, but it won't last long. It'll decline, and it will decline more as the more resistant variants spread," said Moore, who was not involved in the new research.
The immunity landscape in the United States is more complex than ever. According to CDC data, roughly two-thirds of Americans have completed at least their primary series of Covid-19 vaccines. And data from blood tests shows that almost all Americans have some immunity against the virus, thanks to infection, vaccination or both.
A new preprint study from researchers at Harvard and Yale estimates that 94% of Americans have been infected with the virus that causes Covid-19 at least once, and 97% have been infected or vaccinated, increasing protection against a new Omicron infection from an estimated 22% in December 2021 to 63% by November 10, 2022. Population protection against severe disease rose from an estimated 61% in December 2021 to around 89%, on average, this November.
All of this means the US is in a better spot, defensively at least, than it ever has been against the virus -- which is not to say that the country couldn't see another Covid-19 wave, especially if a new variant emerges that is very different from what we've seen, if immunity continues to wane or if behavior shifts dramatically.