1 chart shows how quickly a booster shot can improve immunity, with some extra protection in just a few days

• Booster shots are being offered to all US adults.
• Studies suggest boosts can improve protection against symptomatic COVID-19, at least temporarily, with benefits beginning to accrue after a few days.

Two shots are good, but when it comes to preventing symptomatic COVID-19 infections, three may be even better.

It's tough to say what the long-term benefit of coronavirus booster shots will be, because no one has had COVID-19 boosts in their system for long. But immunologists generally agree that boosting people many months after they are first vaccinated gives a jolt to the immune system.

Early-stage booster shot studies from around the world are starting to suggest that the protection people get when they have an additional shot months after their first vaccination course is more potent, sending antibody levels soaring to new heights, and (at least temporarily) bolstering protection against COVID-19.

Take the data available from Pfizer's booster shot trial, which recruited roughly 10,000 people who were fully vaccinated with two shots of the company's COVID-19 vaccine. Trial participants were split into two groups: about 5,000 got boosted, while the other 5,000 recieved a fake (placebo) jab with no extra vaccine inside.

It didn't take long to spot the difference between boosted and unboosted vaccinated people in the trial. As Pfizer showed in the chart below, which was recently shared with advisors to the Centers for Disease Control and Prevention, booster shot recipients start benefitting from increased protection against symptomatic coronavirus infections just a few days after their third dose.

"If you look at the blue, it's what happens after you get the third dose booster shot," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said on Monday during a White House briefing, explaining how to read the chart, which tracked the number of symptomatic coronavirus infections among Pfizer's trial participants for 100 days after they were jabbed.

Fauci pointed out the people who had boosts in Pfizer's trial (blue line) had a far lower incidence of COVID-19 than those with just two shots who received a fake, placebo shot (red line).

"Unequivocally, effect of the booster shot," Fauci said.

Improved protection against symptomatic COVID-19 ramps up in about a week or two

According to Pfizer, the relative vaccine efficacy of the booster shot is 95% here. That means 95% of the people in the Pfizer trial who got sick with COVID-19 were vaccinated but not boosted, while the other 5% of sick people were boosted. (Scientists started tallying symptomatic COVID-19 cases starting seven days after the boosts were given out, and continued doing so until day 100.)

But you don't have to take Pfizer's word for it.

A landmark National Institutes of Health trial mixed and matched people who'd gotten Pfizer, Moderna, and Johnson & Johnson vaccines into different booster groups and found that no matter which combination of initial vaccine and booster people had, their antibody levels soared (suggesting better protection against infections), with improvements peaking at around two weeks post-boost.

Another study conducted in the UK (which is still under scientific review) found booster shots given out starting in September, for both Pfizer and AstraZeneca recipients, increased vaccine effectiveness in people over age 50 to more than 93%, a dramatic improvement. And data from Israel suggests that at 12 days post-boost, the rate of infection among booster shot recipients plummets to 11 times lower than that of fully vaccinated people, and the rate of severe illness goes down more than 19-fold among booster recipients.

Across the board, a trend is becoming clear. Booster shots amp up our immune response, making it less likely boosted people will get sick with COVID-19 — at least for a while.

But vaccine experts also stress that it's important to remember the blockbuster benefits of a booster shot will likely wane over time. Boosts may be a good idea before people gather for the holidays, but they won't last forever.

"It's just kind of a normal part of our immune system that after vaccination, or exposure, our antibody levels go way up, and then they come down over time," Dr. William Moss, executive director of the International Vaccine Access Center at Johns Hopkins University, told Insider. "They have to come down, or else our blood would just fill with antibodies."

Fortunately, vaccination also promotes a more enduring memory immune response that lasts for many months longer than the antibodies do, which means that even if vaccinated people do get a little sick , they are generally well-protected against the worst that COVID-19 can do.