The majority of U.S. adults — including most of America's seniors — are heading into the holiday season and an expected rise in cases without the maximum level of vaccine protection against the virus.

Driving the news: Only 36% of American seniors — and just 16% of the population 18 and older — have received the updated booster shot that became available in September.

Why it matters: The vaccines' effectiveness wanes over time, meaning that even vaccinated people who haven't received a shot in awhile may not be well-protected against serious infections.

The big picture: All of the data that's come out over the last two years says that the vaccines are effective. The problem is twofold: the effectiveness changed as the virus evolved, and it also fades if you haven't had your last shot in a while.

• The lesson is that the vaccines work, getting a booster shot is important — especially for older, vulnerable people — and people who haven't received a shot recently are at significantly higher risk than if they had.
• The vaccines' effectiveness against severe disease and death holds longer than protection against getting sick with the virus, but it still fades significantly.
• COVID hospitalizations are currently on the rise, especially among seniors, at least partially reflecting the lackluster uptake of booster shots.

Driving the news: Recent CDC data has quantified the value of the updated booster shots, which are targeted against both the original virus strain and the Omicron variant.

• A pair of studies released earlier this month presented a mixed picture of the shots' effectiveness against hospitalization. One found that the updated shot was 57% effective against hospitalization when compared with no vaccination.
• Compared to receipt of the last dose five to seven months earlier, the updated shot was 38% more effective at preventing hospitalization, the study found — but if the last dose was 11 months earlier or more, the updated shot was 45% more effective.

The second study found a higher effectiveness among adults 65 and older. Compared to unvaccinated people, the updated vaccine was 84% effective against hospitalization.

• Compared to people who had received two or more doses of the original shot only, the updated one was 73% effective.
• Relative vaccine effectiveness is impacted by the fact that so many Americans — including unvaccinated people — have already been infected by COVID, meaning they may have some natural immunity.

Yes, but: The shots are much less effective at preventing routine infections, and some scientists say the boosters' value may be more limited in younger populations who generally aren't at risk of severe cases anyways.

• “There’s a world of difference in what's happening to younger healthy people and what’s happening to older, sicker people," said Cornell virologist John Moore. "If you’re in a vulnerable population, it absolutely makes sense to be up to date with your boosters."

Zoom in: Many Americans don't think they need the updated shots.

• In a recent KFF poll, 44% of vaccinated people surveyed who had not received an updated booster dose said they don't think they need it, including nearly two-thirds of Republican or Republican-leaning respondents.
• Another 37% said they didn't think the benefit was worth it.

Between the lines: A recent report from the U.K. Health Security Agency that focused on older Omicron variants found that six to eight months after receipt of a second dose of Pfizer or Moderna vaccine, protection against hospitalization and mortality had fallen to around 50% compared to unvaccinated people.

• That's compared to around 80% effectiveness for the first three months after the shot.
• People who had received a third dose of either vaccine were slightly better off, with about 60% effectiveness against hospitalization nine months out.

What we're watching: The virus has evolved even since the updated boosters were released, and the threat of new variants that can further escape the vaccines' protection hasn't gone away.