It’s possible to get COVID-19 after vaccination. But early Idaho data show how rare it is.

There is mounting evidence that COVID-19 vaccines are very effective against the coronavirus. The U.S. Centers for Disease Control and Prevention this week published a study of frontline workers in eight cities — including Portland and Salt Lake City — that found Pfizer and Moderna vaccines were 90% effective in preventing infection.

But what about people who get vaccinated and still contract COVID-19? What happens to them?

Early data from Idaho offer some insights into those “breakthrough” cases — when the virus breaks through the protective wall of immunization.

Idaho health officials on Tuesday said they have identified 97 cases of people who tested positive for COVID-19 at least two weeks after receiving their second dose of a Pfizer or Moderna vaccine.

That is 0.03% percent of the more than 289,000 Idahoans who had been fully vaccinated as of this week, according to CDC data.

“Because no vaccine is 100% effective, we did expect that we would see some instances of people being exposed to the virus after being vaccinated, and then test positive for the virus that causes COVID-19,” said Kathy Turner, chief of the Idaho Division of Public Health’s Bureau of Communicable Disease Prevention.

Public health officials say it’s important to continue wearing masks, keeping a distance from others and following other guidelines even after being vaccinated. Not only does it take a while to build immunity after receiving a shot, scientists still aren’t sure whether or not vaccinated people can transmit the virus.

Of those 97 people with “breakthrough” infections, half of them had no symptoms, Turner said. They’d been tested for work or as a precaution, and otherwise wouldn’t have known they had the virus, she said.

“Among the people who did experience illness, 80% of them had either very mild symptoms (similar to allergies or a head cold) or they experienced moderate illness, so similar to flu-like symptoms in which most people feel really crummy for a couple of days … and then feel a little bit better,” Turner said.

Three of the 97 people who tested positive after being fully vaccinated did end up in the hospital, she said. They had “pre-existing, serious medical conditions that put them at very high risk for severe disease,” she said.

And in this early cohort of people infected after vaccination, zero died.

The question of vaccine effectiveness in real-world situations is top of mind among public health officials. They eagerly await more data on whether variants of the SARS-CoV-2 virus can slip past the immune defenses that people build after receiving the Pfizer, Moderna or Janssen/Johnson & Johnson vaccines.

Idahoans are just now starting to approach their “fully vaccinated” status with the newer Janssen/Johnson & Johnson vaccine. So the state has no data yet on breakthrough infections with that one-shot vaccination.

The CDC hasn’t yet shared national data on breakthrough infections, Turner explained. She told the Idaho Capital Sun that she has asked the CDC whether such data are forthcoming.

“The CDC is collecting these data from all states and jurisdictions into a database, but not a lot has been released yet,” she told reporters Tuesday.