From the pandemic’s earliest days, scientists have counted on COVID-19 vaccines to lead us out of the international health emergency. But they’ve also been aware that the immunity provided by vaccines might not last long.

The surmise that vaccine-induced immunity would wane quickly was based on previous experience with other coronaviruses – especially four species of seasonal coronavirus that have circulated for as long as modern medicine has been paying attention.

Those four members of the coronavirus family differ in many ways from the SARS-CoV-2 virus that causes COVID-19, not least because they lead to nothing worse than a common cold. But scientists assumed their family resemblances would be revealing.

A 1990 research effort that involved infecting British volunteers with those coronaviruses found that after a year, most still had elevated antibody levels. Those extra antibodies didn’t shield them from reinfection when they were deliberately exposed again, but none developed cold symptoms and most cleared the virus quickly.

Another study published in 2020 tested a small group of healthy people at least twice a year for more than 12 years. It found that after being infected by any of those four coronaviruses, immunity rarely lasted much longer than 12 months. In some instances, reinfection occurred in as little as six months.

The remarkable effectiveness of the COVID-19 vaccines made by Pfizer-BioNTech and Moderna made some scientists optimistic that the pattern might change.

In a preliminary report posted online last month, researchers at the La Jolla Institute for Immunology found that people who received the Moderna vaccine developed the levels of immunity that augured well for lasting protection.

Even better were the specific types of immunity the researchers found, including the long-lasting T-cells that continue to generate protective antibodies.

“I thought from the start there’d be a 50-50 chance we’d need vaccines in a year,” said Shane Crotty, a vaccine researcher at the La Jolla Institute who has co-written two studies on SARS-CoV-2 immunity. “But we found a lot of evidence of durable immunity that would probably last for years in most vaccinated individuals.”

Even so, scientists have been watching closely for the expected dropoff in vaccine-induced immunity.

The National Institutes of Health launched a raft of studies aimed at identifying “correlates of immunity” – antibodies and other physiological measures that could indicate whether a person is protected against a disease. Seeing those correlates of immunity dip below a certain threshold could provide early warning of reinfection risk.

The Centers for Disease Control and Prevention, meanwhile, is tracking several large groups of people that were among the first to receive COVID-19 vaccines, including health care workers and people who live in group housing. These groups are the proverbial canaries in the coal mine, indicating to researchers when boosters might become necessary.

So far, Israel has generated some of the most persuasive evidence of waning vaccine immunity.

With 68% of residents 12 and over fully vaccinated against COVID-19, the country appeared to have coronavirus outbreaks almost completely suppressed by the beginning of summer. Since June, however, its cases have been doubling every seven to 10 days.

More than half of Israel’s new cases have occurred in fully vaccinated people, a sign that vaccine-induced immunity was beginning to wane.

That suspicion was borne out by the findings of a preliminary study posted online last month. Researchers at Maccabi Healthcare Services, one of Israel’s two state-run HMOs, found that during June and July, Israelis who got their first dose of the Pfizer-BioNTech vaccine in January were more than twice as likely to be reinfected than their counterparts who got their first jab in April.

Many of those with so-called breakthrough infections were older Israelis and people with health conditions that make them more susceptible to the virus. And nearly all of the infections involved the highly transmissible delta variant, which replicates much more quickly than its predecessors and may be able to get past the immune system’s defenses while it’s still ramping up.

But the likelihood nevertheless appeared high that, in some people at least, vaccine-induced immunity had weakened.

Crotty said he suspects the delta variant is responsible for undermining the vaccine-induced immunity that had once seemed so promising.

“Delta looks a lot tougher, and that has thrown a lot of uncertainty in there,” he said. But he’s still not convinced that booster shots need to be in everyone’s future.

The reported U.S. plan to offer boosters to people eight months after their second dose “is a better-safe-than-sorry kind of decision,” Crotty said.

“Are boosters required, or critically needed? I don’t think the data support that,” he said. “Will boosters help? Will protective immunity be better? Yeah! They top off antibody levels, and the trials look great.”

Dr. Francis Collins, director of the National Institutes of Health, acknowledged Tuesday that Israeli data on breakthrough infections has powerfully influenced the thinking of American health officials.

“The people who got immunized in January are the ones that are now having more breakthrough cases,” Collins said during an interview on “The Hugh Hewitt Show.” While most have not become very ill, he added, “you’re starting to see a little bit of a trend toward some of those requiring hospitalization.”