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Health officials in Oregon are investigating four cases where people contracted the coronavirus after receiving both doses of the vaccine.
The symptoms of the four individuals range from asymptomatic to mild, Oregon's Health Authority said in a release on Friday.
"We are working with our local and federal public health partners to investigate and determine case origin. Genome sequencing is underway, and we expect results next week." the agency said.
These individuals were referred to as "breakthrough cases" because they got sick with the novel virus at least 14 days after receiving the second dose of the Covid-19 vaccine.
The Health Authority said that such cases were not "unexpected" during the pandemic. "Clinical trials of both vaccines presently in use included breakthrough cases. In those cases, even though the participants got Covid-19, the vaccines reduced the severity of illness," the agency said.
Two of the cases were discovered in Yamhill County, and two others are in Lane County. Local health officials were now looking to determine the origin of these cases to provide the public with more information.
The announcement in Oregon comes as there was a breakthrough case revealed in North Carolina, according to NBC affiliate WCNC-TV in Charlotte.
Health officials in the state told the outlet that the person experienced mild symptoms and was not hospitalised from Covid-19.
Pfizer and Moderna are the only two companies that have received emergency use authorisation for their Covid-19 two-dose vaccines from the Food and Drug Administration (FDA). Studies indicate they each are about 94 per cent effective against Covid-19 and reduce the severe symptoms one might experience after contracting the novel virus.
The Centers for Disease Control and Prevention (CDC) has updated its guidance to state that people who are fully vaccinated no longer need to quarantine if exposed to someone with Covid-19. People are typically asked to isolate themselves for one to two weeks if exposed.
But those who are fully vaccinated, meaning it's been two weeks since receiving the second dose of the vaccine, no longer need to quarantine as long as they've not developed symptoms.
People who are vaccinated are asked to still follow other coronavirus safety measures such as wearing masks, practising social distancing, and avoiding large crowds.