Breakthrough infections and booster shots: what you need to know

Federal health authorities have recommended booster shots for all adults who are fully vaccinated against Covid-19 with either the the Pfizer or Moderna vaccines, citing evidence that suggests breakthrough infections could become more common over time.

The plan has faced scrutiny. Some US scientists have said data was insufficient to support the decision, and vaccines remain highly effective against severe disease requiring hospitalization and death. The World Health Organization harshly criticized US leaders for using vaccines to provide a third shot to people, even as most around the world haven’t had one.

Health officials also said it was possible people who received the single-dose Johnson & Johnson vaccine may need a booster shot, but they said they were waiting for more data.

Here’s what you need to know about the latest on booster shots and breakthrough infections:

What did we learn about breakthrough infections this week?

First, it is important to note that the current devastating wave of Covid-19 in the US is a “pandemic of the unvaccinated”, with the overwhelming majority of hospitalizations among patients who are unvaccinated. Deaths of vaccinated individuals remain exceedingly rare.

Federal authorities said new data suggests mild to moderate breakthrough infections among people who received messenger RNA vaccines (the Moderna and Pfizer Covid-19 shots) could become more common the longer since people received their vaccinations.

This is “waning” immunity, and prompted their move this week to offer booster shots for individuals who received their second Pfizer or Moderna shot eight months ago.

When administration officials made this announcement, the CDC published three reports, which officials said buttressed their decision. These studies provided real-world insight into how vaccines protect against infection and hospitalization.

The first was a large study among nursing home residents, who tend to be frail and elderly. The study found vaccine efficacy against infection waned, coinciding with the time when the Delta variant grew more dominant.

Before Delta, mRNA vaccines provided 74.7% protection against infection for this demographic. After Delta, these vaccines provided protection against infection 53.1% of the time. The study tells us only about infection, not the severity of illness, because the study’s authors only documented infected versus Covid-free patients.

In a second study, researchers cross-referenced databases with hospitalizations and vaccination status in New York City. The study found vaccine efficacy against infection declined from 91.7% to 79.8% around the time Delta came to dominance. However, they also found vaccine efficacy against severe disease requiring hospitalization remained “relatively stable”.

“Covid-19 vaccines were highly effective against hospitalization,” more than 90%, “for fully vaccinated New Yorkers”. That held true for the Delta variant.

A third, multi-state study looked at more than 1,100 patients and found there was “no decline in vaccine effectiveness against Covid-19 hospitalization” over a 24-week period. However, it did find that vaccine efficacy was lower in people with weakened immune systems.

What do these studies tell us?

Taken together, these studies appear to show vaccine efficacy against infection declined, which scientists expected; but protection against severe disease requiring hospitalization remains high.

This explains why a separate analysis showed the vast majority of patients hospitalized in the current, fourth wave – more than 94% – are unvaccinated, and deaths among fully vaccinated individuals are “effectively zero”. Right now, 51% of all Americans are fully vaccinated, though coverage varies widely depending on location.

Despite the high degree of protection against hospitalization these studies found, the Biden administration said immunity “could” wane in the future, even for severe outcomes, and emphasized boosters would help Americans “stay ahead of this virus”.

“Even though this new data affirms that vaccine protection remains high against the worst outcomes of Covid, we are concerned that this pattern of decline we are seeing will continue in the months ahead,” said the US surgeon general, Dr Vivek Murthy. That pattern of waning immunity “could lead to reduced protection against severe disease, hospitalization and death”.

However, even as many scientists predicted boosters would be necessary, some questioned whether the data supported their distribution now. For example, evidence presented by the administration did not show whether a pattern of waning immunity for mild and moderate disease will translate to waning immunity against hospitalization and death.

“They seem to imply, at least to me, that the trend of decreased vaccine effectiveness for mild to moderate disease could extend to moderate to severe disease,” said Dr William Moss, professor at Johns Hopkins University Bloomberg School of Public Health and executive director of the International Vaccine Access Center. “That’s an assumption they’re making, we don’t have evidence of that.”

The White House chief medical adviser, Dr Anthony Fauci, also presented evidence that antibody levels go up significantly after a booster shot. That would be expected. However, how long that heightened immunity might last is unknown.

This has prompted concern that increased protection against mild to moderate disease could be only temporary, and perhaps not worth the effort and supplies when billions of people globally lack access to vaccines.

As Anna Durbin, a vaccines researcher at Johns Hopkins University, told Stat News, “We cannot keep [boosting] and say: ‘We’re going to prevent colds in everybody.’”

What does that mean for keeping myself safe?

First, if you have not already, get vaccinated. It is the best way to protect yourself and the people around you from Covid-19 infection, hospitalization and death.

Vaccines have been reviewed by multiple independent scientific panels, and before boosters are rolled out in the US in September, vaccines by Pfizer and Moderna will be reviewed again by the Food and Drug Administration and a CDC advisory panel. Though there has been debate among scientists over whether boosters are really needed for all Americans, no safety concerns have been raised.

If you’re fully vaccinated, your own level of risk of breakthrough infection will depend on your unique circumstances. People with moderately to severely weakened immune systems are already eligible for a third mRNA vaccine shot. Elderly people may also be at heightened risk for a breakthrough infection, and may experience more severe disease.

The good news is old-fashioned public health measures (the ones with which everyone is now familiar) still work. Masking, social distancing and hand-washing can significantly reduce potential virus exposure, alongside the strong degree of protection afforded by the vaccine. Children under 12, who are not yet eligible for the vaccine, should practice masking in schools, according to the CDC.

People who live in areas of high transmission, which at this point is most of the US, are also more likely to be exposed to the virus on a regular basis, which could lead to a breakthrough infection. Masking and social distancing in public are most important in these areas.

As more people are vaccinated, breakthrough infections will represent a higher proportion of overall infections. This is not a sign vaccines are failing.

What does this mean for the future of the pandemic?

“Our number one, two and three priority is vaccinating the unvaccinated,” said Moss. Booster shots will not significantly reduce the spread of Covid-19 – but getting new people vaccinated will.

That means vaccinating people with initial doses in the US and globally is important, because anywhere Covid-19 is spreading widely, the virus can evolve new variants and evade the protection vaccines provide.

Even though the US have committed to donating more than 600m vaccine doses globally, it is a “drop in the bucket” compared with the billions needed. That leaves the world vulnerable to new variants.

The virus will be with us – probably for the rest of our lives. Indeed, many people will be exposed to Covid in their lives, perhaps multiple times.

“We’re not going to eliminate, to eradicate, this virus,” said Moss. The long-term, population-level goal is to prevent severe disease, said Moss, and the safest way to do that is through vaccination.