Protection from a third mRNA COVID-19 vaccine dose declines over several months but remains substantially stronger than protection after two doses, according to new U.S. data collected as the Omicron variant became dominant.

A Centers for Disease Control and Prevention study of more than 240,000 emergency department and urgent-care encounters and more than 93,000 hospitalizations found that vaccine effectiveness after a third dose was 87% against emergency or urgent-care visits during the first two months, falling to 66% by four to five months. Protection against hospitalization was 91% during the first two months after a third dose and 78% at four months or more.

The findings provide some of the clearest U.S. evidence yet that booster protection also wanes with time, even though a third dose continues to provide materially stronger protection against severe disease than a two-dose series. The data arrive as federal health officials consider how long current booster recommendations will remain adequate during a rapidly changing pandemic.

Omicron changed both exposure and immune escape

The study spans August through January across 10 states, allowing researchers to compare periods dominated by Delta with the period in which Omicron rapidly displaced it. Omicron’s mutations make it more capable of causing infections in people with vaccine- or infection-derived immunity, raising the importance of measuring protection against severe outcomes rather than infection alone.

CDC genomic surveillance shows how quickly the environment changed. A separate MMWR report estimated that Omicron rose from a small share of U.S. infections in early December to 99.2% by the week ending January 22. That transition means vaccine-effectiveness estimates from the Delta period cannot simply be assumed to apply to the current wave.

The new analysis found protection declining with time after both second and third doses. Because relatively few people had received boosters more than five months earlier, estimates at longer intervals are less precise. Even so, the direction of the data is consistent enough to raise questions about whether some groups may eventually need additional doses.

Earlier studies already showed the value of a third dose

A January VISION Network analysis found that during Omicron circulation, three mRNA doses provided substantially stronger protection against emergency visits and hospitalization than two doses. Those results helped establish that boosting restores protection that had declined after the primary series.

Research focused on symptomatic infection points in the same direction. A JAMA study using pharmacy testing data found that three doses were associated with greater protection against symptomatic Omicron infection than two. The absolute effectiveness against infection was lower than had been observed against earlier variants, but the incremental benefit of the booster was clear.

Together, the studies suggest a two-part pattern: immunity can be strengthened meaningfully with another dose, but that increase does not remain fixed indefinitely. Antibody levels naturally decline after vaccination, and an immune-evasive variant such as Omicron places additional pressure on the protection generated by the original vaccine formulation.

Booster policy has expanded rapidly

Federal recommendations have evolved as evidence accumulated. On January 5, the CDC recommended Pfizer boosters for adolescents ages 12 through 17 and shortened the interval after the primary Pfizer series to five months. The agency has also encouraged boosters for adults, with schedules varying by vaccine and immune status.

The FDA summarized the expanding booster framework in January congressional testimony, noting that it had expanded Pfizer booster eligibility to ages 12 through 15, shortened the Pfizer interval and authorized an additional primary-series dose for certain immunocompromised children ages 5 through 11. Moderna’s booster interval was also shortened to five months for eligible adults.

Those actions were taken before the newest waning data were available. The central policy question now is whether declining effectiveness several months after a booster should lead to another dose for everyone, for older and medically vulnerable adults first, or not at all until more evidence accumulates.

Severe-disease protection remains the key benchmark

Declining vaccine effectiveness does not mean the vaccines suddenly stop working. A reduction from 91% to 78% effectiveness against hospitalization still represents substantial protection, and the estimates compare vaccinated people with those who are unvaccinated. Risk also varies by age, health status, prior infection and time since vaccination.

The public-health value of another booster would therefore depend on more than the existence of waning. Officials must consider how quickly protection falls, which populations experience the largest absolute increase in hospital risk, how long an additional dose restores immunity, and whether repeated boosting with the current formulation is the best strategy as the virus evolves.

The timing also matters operationally. Many Americans received boosters in the fall, meaning larger numbers will reach four, five and six months after their third dose during the spring. If the decline observed in the current study continues, health agencies will face pressure to provide clearer guidance before the next major wave.

For now, the evidence reinforces the value of receiving a recommended booster rather than relying indefinitely on a two-dose series. It also removes any assumption that the third dose creates permanent protection. Omicron has made COVID-19 vaccination a moving target, and the latest U.S. data suggest that duration of protection will be as important to future policy as the initial response to each dose.