The Biden administration reportedly plans to announce all fully vaccinated Americans should get a booster shot eight months after their last dose to bolster immunity against COVID-19. The news comes amid a massive summer wave of infections across the country, even in the highly immunized Bay Area, and around the world driven by the highly contagious delta variant of the virus.

Q When will this take effect?

A It’s unclear. Reports are based on unnamed sources in the Biden administration who said an announcement could come as early as this week. But the U.S. Food and Drug Administration still would have to authorize additional doses.

FDA spokeswoman Alison Hunt said the FDA and other federal health agencies “are engaged in a science-based, rigorous process to consider when boosters might be necessary” and deferred questions about the administration plan to the White House, which did not respond.

Q U.S. Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said this is a pandemic of the unvaccinated, with infections and especially hospitalizations and deaths overwhelmingly among those who haven’t had the shots. Why then would the fully vaccinated need another one?

A Administration officials and other health experts have been nervously eyeing outbreak data and studies for signs that the vaccines’ effectiveness may be waning or that variants such as delta can evade their protection. Of particular concern is what has happened in Israel, a global leader in vaccination now battling a delta-driven infection wave. As in the U.S., those infections and particularly hospitalizations have mainly been among Israel’s unvaccinated but also among the vaccinated. So Israel, which used the Pfizer vaccine, has begun giving booster shots to older residents and those with weakened immune systems or sensitive jobs.

“Watching Israel allowed us to understand more that people who’ve been vaccinated can get breakthrough infections, and people who get breakthrough infections can transmit the virus,” said Dr. Kirsten Bibbins-Domingo, professor of epidemiology and biostatistics at the University of California-San Francisco. “Watching some of the laboratory information coming out of Israel suggests that perhaps there may be waning immunity.”

Q Will the boosters be for those who received all three of the vaccines authorized in the U.S. — Pfizer, Moderna and Johnson & Johnson?

A That appears to be the administration’s aim, according to a New York Times account. But the regulatory path for authorization is murky. Pfizer and Moderna use a new two-shot messenger-RNA technology to trigger an immune response to the proteins the virus uses to invade human cells. The one-shot Johnson & Johnson vaccine uses a somewhat older adenovirus technology to accomplish that.

When the U.S. last week recommended booster shots for people with weakened immune systems, it was for those who had the mRNA vaccines, not the J&J shot. There are also indications the Pfizer shot’s immunity may wane faster than Moderna’s. Recent data found Moderna’s vaccine about 93% effective against infection after six months, similar to the 94.5% efficacy reported in its earlier clinical trial. But Pfizer’s vaccine was 84% effective against infection at six months, down from 96% within two months of vaccination.

“Pfizer and BioNTech have put into place a robust booster research program to ensure that our vaccine continues to offer the highest degree of protection possible,” Pfizer spokeswoman Keanna Ghazvini said.

There also have been signs the J&J shot doesn’t hold up so well. A CDC study of a massive outbreak in Cape Cod over the July 4th holiday among mostly vaccinated people found three of the five vaccinated people hospitalized with breakthrough infections had the J&J shot.

Q Will the booster shots be given out by priority group, as initial doses were?

A Federal officials told the New York Times the booster program would likely proceed like the initial vaccination program, in which health care workers and the elderly were prioritized initially. But vaccines are in greater supply now than they were then. It’s unclear whether vaccines would be restricted to those at least eight months since their last shot, which would effectively repeat those earlier prioritizations, and whether first doses would be prioritized over boosters.

Q Are other countries doing this?

A Like the U.S., other countries have authorized booster shots for those deemed at higher risk from a COVID-19 infection, such as those with weakened immune systems. Israel has approved boosters for those 50 and older, people with weakened immune systems and those who work with older people. Germany, France and the United Kingdom have indicated plans to offer boosters to vulnerable populations.

Q The World Health Organization director general and many prominent health officials have called for wealthy, well-vaccinated countries to help vaccinate the rest of the world before giving booster shots to their people. Are boosters selfish?

A There has been much debate about this among health experts. “This is still a global pandemic, and if we’re making global decisions on how to make sure the most people have a benefit, it would favor making sure we have more people vaccinated,” Bibbins-Domingo said. But Dr. Bob Wachter, who chairs the UCSF medical department, said Tuesday on Twitter that there would be “political fallout if we start seeing big upticks in cases” among vaccinated Americans, and the U.S. should do boosters and provide vaccines to the world.

“We need to pursue ‘and’,” Wachter said, “not ‘or’.”