PORTLAND, Ore. (KOIN) — Health officials in Washington said Tuesday they’re investigating reports of residents who tested positive for COVID-19 even after being fully vaccinated.

The Washington State Department of Health said epidemiologists have found evidence of about 102 so-called “vaccine breakthrough” cases for every 1 million fully vaccinated Washington residents since Feb. 1. That’s about .01% of vaccinated people in Washington.

The breakthrough cases have been reported in 18 counties. Most of those who tested positive for COVID-19 experienced mild symptoms, if any, according to state officials. A total of eight people with breakthrough cases needed treatment at hospitals since Feb. 1. Authorities said they’re still investigating two potential cases in which the patients, who were over 80 years old with underlying conditions, died.

Vaccine breakthrough cases are “expected with any vaccine” and large clinical studies found the vaccines reduced the risk of getting the virus by up to 95%, health officials said.

“It is important to remember that every vaccine on the market right now prevents severe disease and death in most cases,” said Umair A. Shah, MD, MPH, Secretary of Health. “People should still get vaccinated as soon as they are eligible, and encourage friends, loved ones, and co-workers to do the same.”

Shah said the breakthrough cases go to show people who have been vaccinated still need to wear a mask and follow other health guidelines to prevent spreading the virus to those who haven’t been vaccinated.

State officials plan to provide regular updates on vaccine breakthroughs starting in late April.