What you need to know about COVID-19 boosters and how long they take to work

COVID-19 booster shots are now authorized for anyone over the age of 18. But experts say it's important to understand that protection doesn't kick in right away.

Although the body starts to increase antibodies within a few days after the shot, it takes two weeks for peak protection to return.

“There is a meaningful increase in antibody titers by one week and peak responses at 2 weeks following mRNA boosting,” said Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston.

More than 32 million people in the U.S. have already received a COVID-19 booster dose. The FDA and CDC say it's safe to mix and match boosters.

COVID-19 booster shots are composed of the same formulation as the current COVID-19 vaccines. But the Moderna vaccine booster is given at half the dose of the primary Moderna series.

Studies show the immunity gained from the primary vaccine series may decline over time -- particularly protection against mild breakthrough infections. Therefore, experts recommend booster vaccination to increase immunity.

“The booster shot will have a similar response as the second dose of the primary series. The highest antibody level will be around 10-14 days and then things will level off. The antibody levels will decline as it would with all vaccines, however the boosters help to stabilize the antibody levels,” said Dr. Simone Wildes, an infectious disease specialist at South Shore Health in Massachusetts.

Public health experts say booster shots could also help slow the spread of COVID-19 within communities. However, to truly curb the pandemic, it's far more important for the unvaccinated to get their initial doses.

The expanded authorization of the Moderna and Pfizer boosters comes right in time for the holidays to allow many more Americans to get their booster shots before the holidays for safer travels and gatherings. The expanded authorization to now everyone over the age of 18 also helps eliminate any confusion on who may receive booster shots and ensures boosters are available to everyone who needs one.

Esra Demirel, MD is an OB-GYN resident physician at Northwell Health-North Shore University Hospital & LIJ Medical Center and is a contributor to the ABC News Medical Unit.