Unforeseen Impact: Why the inoculated get breakthrough infections

A wedding in Oklahoma leads to 15 vaccinated guests becoming infected with the coronavirus. Raucous Fourth of July celebrations disperse the virus from Provincetown, Mass., to dozens of places across the country, sometimes carried by fully vaccinated celebrants. As the Delta variant surges across the nation, reports of infections in vaccinated people have become increasingly frequent — including, most recently, among at least six Texas Democrats, a White House aide and an aide to Speaker Nancy Pelosi. The highly contagious variant, combined with a lagging vaccination campaign and the near absence of preventive restrictions, is fuelling a rapid rise in cases in all states, and hospitalisations in nearly all of them. It now accounts for about 83 percent of infections diagnosed in the United States. 

But as worrying as the trend may seem, breakthrough infections — those occurring in vaccinated people — are still relatively uncommon, experts said, and those that cause serious illness, hospitalisation or death even more so. More than 97 percent of people hospitalised for COVID-19 are unvaccinated. 

“The takeaway message remains, if you’re vaccinated, you are protected,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “You are not going to end up with severe disease, hospitalisation or death.” 

Reports of breakthrough infections should not be taken to mean that the vaccines do not work, Dr. Anthony S. Fauci, the Biden administration’s top pandemic adviser, said on Thursday at a news briefing. “By no means does that mean that you’re dealing with an unsuccessful vaccine,” he said. “The success of the vaccine is based on the prevention of illness.” Still, vaccinated people can come down with infections, overwhelmingly asymptomatic or mild. That may come as a surprise to many vaccinated Americans, who often assume that they are completely shielded from the virus. And breakthrough infections raise the possibility, as yet unresolved, that vaccinated people may spread the virus to others. 

Given the upwelling of virus across much of the country, some scientists say it is time for vaccinated people to consider wearing masks indoors and in crowded spaces like shopping malls or concert halls — a recommendation that goes beyond current guidelines from the Centers for Disease Control and Prevention, which recommends masking only for unvaccinated people. 

The agency does not plan to change its guidelines unless there is a significant change in the science, said a federal official speaking on condition of anonymity because he was not authorised to speak on the matter. The agency’s guidance already gives local leaders latitude to adjust their policies based on rates of transmission in their communities, he added. Citing the rise of the Delta variant, health officials in several California jurisdictions are already urging a return to indoor masking; Los Angeles County is requiring it. 

“Seatbelts reduce risk, but we still need to drive carefully,” said Dr. Scott Dryden-Peterson, an infectious disease physician and epidemiologist at Brigham & Women’s Hospital in Boston. “We’re still trying to figure out what is ‘drive carefully’ in the Delta era, and what we should be doing.” The uncertainty about Delta results in part from how it differs from previous versions of the coronavirus. Although its mode of transmission is the same — it is inhaled, usually in indoor spaces — Delta is thought to be about twice as contagious as the original virus. 

Significantly, early evidence also suggests that people infected with the Delta variant may carry roughly a thousandfold more virus than those infected with the original virus. While that does not seem to mean that they get sicker, it does probably mean that they are more contagious and for longer. 

Dose also matters: A vaccinated person exposed to a low dose of the coronavirus may never become infected, or not noticeably so. A vaccinated person exposed to extremely high viral loads of the Delta variant is more likely to find his or her immune defenses overwhelmed. The problem grows worse as community transmission rates rise, because exposures in dose and number will increase. Vaccination rates in the country have stalled, with less than half of Americans fully immunised, giving the virus plenty of room to spread. Unvaccinated people “are not, for the most part, taking precautions, and that’s what’s driving it for everybody,” said Dr. Eric J. Rubin, the editor in chief of the New England Journal of Medicine. “We’re all susceptible to whatever anyone’s behavior is in this epidemic.” 

Dr. Gounder likened the amount of protection offered by the vaccines to a golf umbrella that keeps people dry in a rainstorm. “But if you’re out in a hurricane, you’re still going to get wet,” she said. “That’s kind of the situation that the Delta variant has created, where there’s still a lot of community spread.” 

For the average vaccinated person, a breakthrough infection is likely to be inconsequential, causing few to no symptoms. But there is concern among scientists that a few vaccinated people who become infected may go on to develop long COVID, a poorly understood constellation of symptoms that persists after the active infection is resolved. 

Much has been made of Delta’s ability to sidestep immune defenses. In fact, all of the existing vaccines seem able to prevent serious illness and death from the variant. In laboratory studies, Delta actually has proved to be a milder threat than Beta, the variant first identified in South Africa. 

Whether a vaccinated person ever becomes infected may depend on how high antibodies spiked after vaccination, how potent those antibodies are against the variant, and whether the level of antibodies in the person’s blood has waned since immunisation. 

In any case, immune defenses primed by the vaccines should recognise the virus soon after infection and destroy it before significant damage occurs. “That is what explains why people do get infected and why people don’t get seriously ill,” said Michel C. Nussenzweig, an immunologist at Rockefeller University in New York. “It’s nearly unavoidable, unless you’re going to give people very frequent boosters.” 

There is limited evidence beyond anecdotal reports to indicate whether breakthrough infections with the Delta variant are more common or more likely to fan out to other people. The C.D.C. has recorded about 5,500 hospitalisations and deaths in vaccinated people, but it is not tracking milder breakthrough infections. 

Mandavilli is a reporter focusing on science and global health for NYT©2021 

The New York Times