What does waning COVID-19 vaccine immunity mean for the vaccinated?

As COVID-19 vaccine manufacturers await U.S. Food and Drug Administration approval on booster shots, millions of fully vaccinated Americans wonder about the next step forward amid the ongoing pandemic. 

Coronavirus cases jumped this summer as the highly transmissible delta variant spread, packing hospitals and leading officials to reverse guidance on face masks.

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However, more than one-third of Americans eligible for the shots aren't yet fully vaccinated, according to data from the U.S. Centers for Disease Control and Prevention (CDC). 

Although the agency reports that 182 million are fully vaccinated – with more than 212 million receiving at least one dose – immunity against the mutating virus remains a concern. 

Some research has shown that coronavirus vaccines have lost their effectiveness over time, though experts have concluded that boosters were not necessary for most younger and healthier Americans without high-risk jobs.

In a recent and controversial study using data from both the U.S. and Israel in "the context of the delta variant," Pfizer Inc. said there was a "strong effect of waning immunity in all age groups after six months," supporting the company's request for a third dose to be given about six months after the second dose in eligible individuals 16 years and older.

The company – which awaits the FDA's judgment – said immunity against milder infection wanes approximately six to eight months after the second dose.

The CDC said real-world data shows protection against severe illness, hospitalizations and deaths is holding strong. 

However, in one recent study, protection against infection fell as the delta variant hit: It was 91% in the spring but down to 78% in June and July. 

In addition, the CDC has reportedly seen a hint that for people 75 and older, protection against hospitalizations slightly declined over the summer.

Moderna Inc. has also posted its own study of waning immunity that compared approximately 14,000 people who had received the first dose a year ago with another 11,000 vaccinated around eight months ago. 

The company concluded that the more recently vaccinated group had a 36% lower rate of "breakthrough" infections compared with those vaccinated longer ago.

The U.S. already offers an extra dose of the Pfizer-BioNTech or Moderna vaccines – at least 28 days after getting the second shot – to people with severely weakened immune systems.

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Nevertheless, scientists have been divided over the use of boosters and who should receive them, as well as the evidence for waning immunity and what should be done about it.

The New York Times pointed out in August that vaccine makers "have an incentive to promote [waning immunity], because booster shots will bring them big profits," and that the "exaggerated discussion of waning immunity contributes to vaccine skepticism."

Many argue that getting all Americans vaccinated will help thwart future viral mutations and eliminate the need for boosters or new vaccines.

The Atlantic and Nature both said that while vaccines don't last forever, the body's cellular "immune memory" creates durable protection against severe disease – for a while. 

"Despite some shifting numbers, neither our vaccines nor our immune systems are failing us, or even coming close. Vaccine effectiveness isn’t a monolith, and neither is immunity. Staying safe from a virus depends on host and pathogen alike; a change in either can chip away at the barriers that separate the two without obliterating them, which is exactly what we’re seeing now," The Atlantic said.

The publication added that breakthroughs remain fairly uncommon, pointing to CDC reports that show Moderna and Pfizer-BioNTech vaccines were blocking infection rates at up to about 90% in the spring and still around the 60s and 70s.

"Early reports, including Moderna’s and Pfizer’s original study estimates, put the vaccines’ efficacy against symptomatic illness in the range of 90 to 95%. More recent studies now document rates in the 80s, even when facing off against delta – a variant for which the vaccines weren’t originally formulated," The Atlantic wrote.

Globally, Nature said, there is currently "no indication that the rates of severe illness among the vaccinated are spiking in any appreciable way."

The outlets highlighted that there are still a lot of unknowns, especially with the introduction of the delta variant.

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Speaking at a coronavirus summit on Wednesday, President Biden assured that the U.S. would continue to lead the world in its response to COVID-19.

Nearly 679,000 Americans have died since the beginning of the pandemic last year, according to the John's Hopkins Coronavirus Resource Center.

The Associated Press contributed to this report.