What does it mean to be ‘fully vaccinated’ against COVID-19?

As evidence grows that the delta and omicron variants of the coronavirus are causing breakthrough infections in people who were once considered “fully vaccinated,” momentum seems to be growing to change the definition of that term to include booster shots.

Some workplaces and college campuses are now mandating that vaccination include boosters. The governor of New York said that state officials planned to change the definition of “fully vaccinated” to include receiving a booster dose, and Britain’s government will not be far behind. The NFL last week issued a booster shot mandate for team staff members who work closely with players.

And the speculation is growing that we may have to get boosters regularly in future years as new variants emerge.

A few months ago, confirming full vaccination status was as simple as showing a card or QR code with proof that the required number of shots had been completed within six months. But in a world of multiple vaccines with varying effectiveness, and a variety of mixing and matching strategies, it will soon be harder to say who is “fully vaccinated.”

A consensus will eventually emerge. But here is what some health experts had to say as another year of living with the pandemic was nearing a close.

What is the official definition of ‘fully vaccinated’?

For now, U.S. health officials say you are fully vaccinated two weeks after your second shot of a two-dose vaccine like Pfizer’s or Moderna’s or after a single-dose vaccine like Johnson & Johnson’s. They have not (yet) expanded that definition to include a booster shot.

At a White House press briefing Wednesday, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said the agency is “continuing to follow” the science around omicron before it decides to expand the definition. However, the agency does recommend that people get booster shots.

So does Dr. Anthony Fauci, the nation’s top infectious disease expert, who said at the same press briefing, “If you are unvaccinated, get vaccinated. And particularly in the arena of omicron, if you are fully vaccinated, get your booster shot.”

Other countries, like Britain and South Africa, also do not require booster shots for someone to be considered “fully vaccinated.”

This was always going to change.

As it became clear that the immunity conferred by the initial rounds of vaccines was waning, Israel announced in October that it would make a booster dose a requirement for its vaccine passport. It was believed to be the first country to do so, though it would not be the last.

In late November, just before omicron fast-forwarded booster programs around the world, the European Union began to discuss adding a nine-month expiration date to its digital certificates, a move it formally adopted this week.

Some of the EU’s member nations, like Austria, had already enacted an expiration date for their residents. In France, where the certificates expire seven months after a second dose, all adults have until Jan. 15 to receive a booster, or their passes will no longer allow them access to places like restaurants and museums.

Do I need a booster shot to fend off omicron?

Early research indicates that the omicron variant is somewhat less vulnerable to the body’s immune defenses. Booster shots help bolster your antibody response, said Michel Nussenzweig, an immunologist at Rockefeller University in New York.

So, yes, you should get that additional jab, said Dr. Jesse Goodman, a former chief scientist at the Food and Drug Administration.

“This booster dose has really protected people better against delta,” he said. “Even without omicron, there’s good reason to get the booster dose.”

You can still get infected even after a booster, but the shot will probably protect you against severe illness or death, he said.

How effective is being ‘fully vaccinated’ at this point?

“It depends on what it is you’re trying to prevent,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center who has previously advised the Biden administration.

A booster is more effective than just the first two shots at preventing hospitalization or death, she said.

Many U.S. public health experts continue to say that the two-dose regimen of Pfizer-BioNTech or Moderna will protect most people against severe illness or death, as the vaccines are intended to do. A preliminary study in South Africa showed that two doses of the Pfizer vaccine were 70% effective against hospitalization, part of the early evidence cited by those supporting boosters.

The first two doses have been effective against infection with the delta variant, but it is not yet clear how well they work against omicron, said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and an adviser to the FDA.

“If the goal is protection against any form of illness, then two doses of the current mRNA vaccine will not protect you as well against mild illness,” he said.

If the definition changes, how does that work?

Like so much else since the pandemic started, expect a period of confusion as a patchwork of local, national and international governments evolve at different speeds. Any places that have so far required proof of vaccination — offices, schools, airlines, concerts, sporting events, entire countries — are likely to soon face questions about how and when to change the rules for admission.

Businesses are already facing uncertainty amid challenges to existing vaccine mandates, and the addition of boosters along with rapidly rising case counts have further confused things.

At BlackRock, an investment management company, the vaccination requirement at its New York offices has not evolved to include booster shots, Brian Beades, a company spokesperson, said Monday. But, as with vaccination policies around the world, he said, “people are thinking about new considerations all the time.”