EXPLAINER: What do we know about booster shots for COVID-19?

U.S. health officials plan to offer COVID-19 booster shots to vaccinated Americans to shore up their protection amid the surging delta variant and evidence that the vaccines’ effectiveness is declining. A look at what we know about boosters and how they could help fight the coronavirus pandemic:

WHY MIGHT WE NEED BOOSTERS?

It’s common for protection from vaccines to decrease over time. A tetanus booster, for example, is recommended every 10 years.

Researchers and health officials have been monitoring the real-world performance of the COVID-19 vaccines to see how long protection lasts among vaccinated people. The vaccines authorized in the U.S. continue to offer very strong protection against severe disease and death.

But their ability to prevent infection is dropping markedly during the delta surge among nursing home patients and others, according to studies the Centers for Disease Control and Prevention released in making the announcement Wednesday.

International data, particularly from Israel where delta hit earlier, also persuaded U.S. health officials to plan for rolling out boosters. The effort could start Sept. 20, pending an evaluation by the Food and Drug Administration and advisers to the CDC.

Already, Israel is offering a booster to people over 50 who were vaccinated more than five months ago. France and Germany plan to offer boosters to some people in the fall. The European Medicines Agency said it too is reviewing data to see if booster shots are needed.

WHEN WOULD THEY BE GIVEN?

It depends on when you got your initial shots. The plan announced Wednesday is for people to get a booster eight months after getting their second shot of the Pfizer or Moderna vaccine.

Officials are continuing to collect information about the one-shot Johnson & Johnson...