The state Department of Health reported a total of 217 COVID-19 vaccine breakthrough cases among vaccinated Washingtonians as of April 3. Vaccine breakthrough occurs when a person tests positive for COVID-19 two weeks or more after receiving the full course of a vaccine. Large-scale clinical studies found that COVID-19 vaccines prevented most people from getting COVID-19 illness. However, the vaccines aren’t 100 percent effective, which means a small number of fully vaccinated people will still get sick with COVID-19, officials said.

The breakthrough cases reported in early April was up from the 102 such cases first reported on March 30. Cases have occurred identified in 24 of Washington’s 39 counties.

By April, more than 1.7 million Washington residents were fully vaccinated. The breakthrough cases, some of which may have led to deaths of the patients, account for one-tenth of 1 percent of the fully vaccinated population, according to the health department (DOH).

The cases are a reminder that “even if you have been vaccinated, you still need to wear a mask, practice social distancing, and wash your hands to prevent spreading COVID-19 to others who haven’t been vaccinated,” said Dr. Umair Shah, the state’s Secretary of Health.

While the majority of individuals with confirmed breakthrough experienced only mild or no symptoms, some people have been hospitalized. And officials said in April that DOH was investigating five suspected deaths of individuals who experienced vaccine breakthrough. They were between 67 and 94 years old and had multiple underlying health conditions. Four were residents of long-term care facilities.

In Washington, the median age of those with confirmed vaccine breakthrough has gotten younger since the first cases were reported, with more people 40 to 59. Some cases showed evidence of variants of the virus.