“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.