MORE OFTEN THAN LESS:

The latest studies show the bivalent boosters effectively protect against severe disease and death. The CDC reported in February that mortality rates among people who received a bivalent booster were 14 times lower than in those who had never been vaccinated and three times lower than in people who received the original COVID vaccination series but no booster. Other early estimates also indicate the bivalent booster increases protection against two of the latest Omicron strains, XBB and XBB.1.5, for at least the first three months after vaccination in people who had previously received at least two of the past monovalent vaccine doses.

Notably, the bivalent shots are especially effective against COVID-associated hospitalization in older adults. But U.S. residents older than age 65 have been getting this booster at lower-than-expected rates--just 42 percent of that population has gotten it--says Carlos del Rio, a clinician and epidemiologist at Emory University. "If you're telling me you're 40 and you have no underlying conditions, it's probably not an urgency to get another booster," del Rio says. "But if you're over 65, that's a problem."

Less is known about how long immunity lasts after receiving the bivalent vaccinations. ACIP's COVID vaccine working group, however, says its information on the original monovalent series and boosters suggests protection against hospitalization starts waning four months after a person receives the dose. "It doesn't go to zero," Schaffner says. "After a period of months, it might go from 90 percent to 70 percent. So from a population basis, there still is substantial protection out there."

On an individual basis, however, waning protection gets more complicated--especially in high-risk groups. Studies have demonstrated that monovalent COVID vaccine effectiveness in certain immunocompromised people, particularly organ or stem cell transplant recipients, is lower than in others. "Wouldn't it be appropriate for [high-risk groups] to receive a spring or summer booster if it's been five or six months since they have received the initial bivalent booster? It's a perfectly reasonable question," Schaffner says.

Protection appears to be restored after people receive additional doses over time, according to ACIP. "We are seeing very, very low rates--far less than 5 percent--of severe disease in people who are immunocompromised, well vaccinated and receive standard antiviral treatments," said committee member Camille Kotton, an infectious disease clinician at Massachusetts General Hospital, during February's ACIP meeting. 

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