CDC study: Booster effectiveness wanes after 4 months, but still significant

The effectiveness of COVID-19 booster shots wanes somewhat after four months but still provides substantial protection against hospitalization, a new study shows. 

The study, released by the Centers for Disease Control and Prevention (CDC) Friday, found that booster shot effectiveness against hospitalization with the omicron variant was 91 percent after two months, but waned to 78 percent after four months. 

Protection against emergency department or urgent care visits declined from 87 percent to 66 percent after four months. It was just 31 percent after at least five months, though the study cautioned that finding was “imprecise because few data were available.”

Vaccine effectiveness was always higher after three doses than after two, so people are still urged to get a booster shot. 

The finding of some waning immunity could inform discussions of the possible need for additional shots.

“The finding that protection conferred by mRNA vaccines waned in the months after receipt of a third vaccine dose reinforces the importance of further consideration of additional doses to sustain or improve protection against COVID-19–associated [Emergency Department/Urgent Care] encounters and COVID-19 hospitalizations,” the study states. “All eligible persons should remain up to date with recommended COVID-19 vaccinations to best protect against COVID-19–associated hospitalizations and ED/UC visits.”

Speaking earlier this week at a press briefing, President Biden’s chief medical adviser, Anthony Fauci, said data is being monitored closely on metrics like protection against hospitalization when it comes to deciding whether a fourth dose is needed. Another shot might not be necessary for everyone, he said.

“There may be the need for yet again another boost — in this case, a fourth-dose boost for an individual receiving the mRNA — that could be based on age, as well as underlying conditions,” he said.

“So I don’t think you’re going to be hearing, if you do, any kind of recommendations that would be across the board for everyone,” he added. “It very likely will take into account what subset of people have a diminished, or not, protection against the important parameters such as hospitalization.”

For the time being, a third shot is still recommended. An earlier CDC study found unvaccinated adults were 23 times more likely to be hospitalized during the omicron wave than adults who were boosted. Unvaccinated people were 5.3 times more likely to be hospitalized than people who were vaccinated but not boosted.

Updated at 2:07 p.m.