"We do not know at this point, but we are collecting data that hopefully will inform us about that," Fauci said when asked by a reporter if people will need to get a fourth vaccination shot six months after their booster or if boosters will eventually be administered every six months to ensure safety from the virus.
Fauci had shown graphs earlier in the briefing that showed the waning potency of the original vaccine regimens over a six-month period, which he said displayed the demand for boosters.
Whether or not the same thing will happen with the booster shots remains to be seen, he said. The expectation is that the period between the second dose of an mRNA vaccine and the third will give the immune response a chance to evolve, which Fauci pointed to as "affinity maturation," and provide longer-lasting protection from the disease.
"I think there's a reasonable chance that the durability for protection following the third dose will be longer than the durability of protection that I just showed in one of my slides," Fauci said. "If that's the case, we may not need to get boosted every six months or so."
But Fauci didn't rule out the chance of a fourth shot or rotating six-month vaccination intervals.
"If so, we will address it," he said. "We will find out the data, we will make it public, and we will act accordingly."
Earlier in the briefing, White House Coronavirus Response Coordinator Jeff Zients said that 1 million boosters were given every day over the weekend, with 36 million total boosters given to date, and 95% of the federal government workforce is compliant with the pending vaccine mandate.
Zients also mentioned that unvaccinated individuals should get their first jab, stressing that unvaccinated people are 14 times more likely to die of COVID-19 if they haven't had the shot and are nine times more likely to be hospitalized.
Fauci also stressed that health officials may change the definition of "fully vaccinated" to include COVID-19 booster shots – if the data suggests it.
The Centers for Disease Control (CDC) last week recommended that all adults aged 18 or older should receive a vaccine booster shot six months after their final shot of a completed vaccine regimen.
Connecticut Gov. Ned Lamont and New Mexico Gov. Michelle Lujan Grisham – both Democrats – have said that the definition of fully vaccinated should now include a booster shot as each advocated for all adults in their state to get the booster shot.