Four people in Oregon have tested positive for coronavirus after receiving both doses of the COVID-19 vaccine, according to state health officials. It's called a 'breakthrough case', when a patient gets sick at least 14 days after their second dose. Such cases showed up during clinical trials and another case was recorded last week in North Carolina. However, experts say even in breakthrough cases, the vaccine can help reduce the severity of the illness. All cases so far have been described as mild or asymptomatic.