The mystery of ‘breakthrough’ infections after shots

Vaccines are the crowning achievement of our battle against the COVID-19 virus.

But they’re not perfect. And our immune systems aren’t either.

About 5,800 of more 78.5 million vaccinated Americans — 0.0074% — have become infected by the virus, according to new data from the U.S. Centers for Disease Control and Prevention. Of these, 396 required hospitalization and 74 people died, the CDC said.

The numbers are showing us how the vaccines are working in real life, beyond carefully controlled clinical trials — and providing the first evidence that they aren’t fail-safe.

“No vaccine works 100% of the time, and there are many reasons

for that,” said Dr. Dean Blumberg, chief of Pediatric Infectious Diseases at UC Davis Health.

But failures are rare — and illnesses in vaccinated people are more likely to be mild than those in unprotected people, he said.

Doctors call these “breakthrough” cases — borrowing a military phrase for an enemy’s offensive assault that penetrates beyond a defensive line.

California hasn’t yet compiled the total number of cases, but is working with local health officers to collect the data, Gov. Gavin Newsom said on Thursday.

Santa Clara County did not report its tally of such cases, but called them “a very small subset” of vaccinated people. San Mateo County has identified 44 cases. Contra Costa County reported 91 cases — 46 asymptomatic and 45 symptomatic. Alameda County did not respond by deadline.

Research at UC Los Angeles found that seven of 14,990 fully vaccinated healthcare workers tested positive two or more weeks after receiving a second dose of either the Pfizer or Moderna vaccines. At the University of Texas, four of 8,121 vaccinated healthcare workers became infected.

While disconcerting to anyone who has struggled to secure a precious shot, such failures are not unique to the coronavirus vaccine, say experts.

Decades of experience show that vaccine protection from a variety of other infectious diseases — such as chicken pox, mumps and flu — isn’t guaranteed, either. Like COVID-19, these illnesses in vaccinated people tend to be milder.

Experts say the cause of such breakthrough illnesses is not completely understood.

“There are probably a variety of different possible explanations for the few cases that we’ve seen in people who are fully vaccinated,” said Dr. Marisa Holubar, infectious disease physician at Stanford Health Care, adding that “some cases may occur before an individual has developed a full immune response to the vaccine.”

Some people may have a delayed immune response, said Dr. Peter Chin-Hong, infectious disease physician at UC San Francisco. Although most people are fully protected after two weeks of vaccination, some people take longer, he said.

“There is a spectrum in the way that people develop antibodies. Maybe some parts of the immune system are a little more sluggish in some people,” he said.

“But it eventually kicks in,” he added, “to prevent the virus from becoming more invasive and causing trouble.”

Agreed Holubar: “Some of the post vaccination cases may occur before an individual has developed a full immune response to the vaccine.”

There are other biological causes. Some people may be genetically wired to mount a less vigorous response.

Others, such as the elderly, lose their defenses over time in a process referred to as immunosenescence. The CDC reported about 40% of the post-vaccine infections were in people 60 or more years of age.

People who have compromised immune systems — due to cancer treatment or dialysis, for instance — may also be vulnerable.

In these groups of people, there are fewer antibodies to fend off the virus. They may also experience a decline in another part of the immune system, involving T cells, so their bodies can’t destroy a rogue virus that slips past their antibodies.

“They may not have as strong a response to the vaccine as people with more intact immune systems,” said Blumberg.

The shape-shifting virus may also be to blame, evolving into new genetic variants that evade a vaccine. When we’re vaccinated, our immune system develops antibodies that recognize specific proteins on a virus. But if a virus accumulates mutations that change its protein structure, we no longer recognize it.

“One big concern is that these variants could escape the immunity that we acquire with immunization,” said Holubar. “So the antibodies that we have against specific targets may not work as effectively.”

Finally, human behavior can contribute to the problem. People who are vaccinated may stop wearing a mask or socially distancing — and if confronted by a high concentration of the virus, even the most sturdy immune system may be unable to mount an adequate defense, said Blumberg.

“If they are around people who are symptomatic with a high concentration of virus, and spend a prolonged period of time around those people, then there’s a really high risk for intense exposure,” he said. “That can overcome even a good immune response.”