With more contagious variants of the COVID-19 virus spreading just as people are starting to feel comfortable reentering society, talk about the benefit of vaccine boosters is amping up.

» READ MORE: I'm vaccinated. Should I still be worried about delta and other variants?

Pfizer is seeking approval for a third dose of its vaccine, given as a booster, and some countries, including Britain and Israel, have already said they will boost vulnerable populations.

Yet the U.S. Centers for Disease Control and Prevention and the Food and Drug Administration have said boosters are not currently necessary for fully vaccinated Americans.

What gives?

We talked to the University of Pennsylvania’s Drew Weissman, an immunologist who helped develop the messenger RNA concept behind Pfizer’s vaccine, to unpack the international argument about COVID-19 boosters.

What is a booster shot?

Vaccines cause our bodies to develop antibodies to protect against a virus. A booster shot is an extra dose that “boosts” immunity by spurring the development of more antibodies.

There are two types of booster shots: The first kind is a follow-up dose that is identical to the initial vaccine, such as the tetanus booster recommended every 10 years. Other vaccine boosters are tweaked from their original form to protect against a new variant. A common example is the influenza vaccine, which is slightly different each season to target the most common current strain of the flu virus.

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The type of boosters developed for COVID-19 may vary. A follow-up dose of the original vaccine would have a faster path to regulatory approval, since vaccine makers have already received emergency use authorization. Tweaked versions of the COVID-19 vaccines could be useful in targeting variants, especially if a variant emerges against which the current vaccines are not effective. No such variant has come about yet — the vaccines have proven effective against the delta and other new variants.

Will booster shots be necessary for the COVID-19 vaccine?

Scientists disagree on whether booster shots are necessary. The CDC and FDA have said there is not currently enough research to suggest that boosters are needed because the vaccines are providing good protection from the original strain and variants, such as delta. Studies have shown the two-dose Pfizer and Moderna vaccines provide greater than 90% protection against infection from the original strain, while the single-shot Johnson & Johnson vaccine provides 66% protection. Nearly all recent COVID-19 hospitalizations and deaths in the United States have been among those who are unvaccinated.

Meanwhile, Pfizer is seeking approval in the United States and Europe for a booster for its COVID-19 vaccine after a study in Israel that found the vaccine’s effectiveness in preventing infection fell to 64% after six months, though cases of severe illness remained low. Pfizer said in a statement that the Israeli findings are consistent with its ongoing Phase 3 clinical trial.

Who is most likely to need a booster?

People who are most vulnerable to severe illness, hospitalization, and death — including the elderly and individuals with autoimmune diseases — would be top of the list for a booster.

Britain plans to administer boosters to people over age 70 beginning in September and Israel has begun offering a third dose of the Pfizer vaccine to at-risk individuals.

There is not enough research to know whether boosters are worthwhile for everyone.

Are there other factors that could influence decisions about COVID-19 boosters?

Of course. Government agencies must consider the full scope of the public health emergency when prioritizing resources and strategizing how best to control the pandemic. Should doses go toward boosting the immunity of people who are already vaccinated — and have considerable protection against current variants — or toward getting more people their first dose?

Getting as many people vaccinated as possible is the best way to reduce the virus’ spread and minimize the likelihood of a new variant against which the current vaccines are not effective, Weissman said.

If a new variant arose that escaped current vaccine protection, vaccination efforts would have to restart with a booster designed to target that variant.

What does the latest Johnson & Johnson risk news mean about vaccine and booster safety?

Through the Vaccine Adverse Event Reporting Systems, a federal monitoring system, officials have identified 100 cases of Guillain-Barre syndrome among the 12.8 million people who have received the Johnson & Johnson vaccine.

“Every drug we take has adverse events. For the [COVID-19] vaccine, they’re in the one-in-a-million range,” said Weissman. People are much more likely to die of COVID-19 than they are to develop a complication from the vaccine, he said.

What does it mean if authorities later decide that everyone should get a booster shot? How can we trust those officials?

It means lawmakers are basing their guidance on science, not politics. Science evolves because scientists are constantly learning through new studies. Scientists make recommendations based on what they know right now. But every new study builds on their knowledge of a disease or virus, and as scientists gain a more complete picture of the problem, their guidance may change.