Sen. Angus King, I-Maine, tested positive for COVID-19 Thursday morning in his home state, a day after he began feeling unwell, his office said.
King is vaccinated and has been taking social distancing precautions as well, he said in a statement.
"While I am not feeling great, I'm definitely feeling much better than I would have without the vaccine. I am taking this diagnosis very seriously, quarantining myself at home and telling the few people I've been in contact with to get tested in order to limit any further spread," he said.
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Coronavirus breakthrough cases like King's -- in which people who've been fully vaccinated get infected -- have been on the rise as the delta variant takes hold nationwide.
Nevertheless, experts say and public health data shows that vaccinated people who become infected are far less likely to become seriously ill or die than unvaccinated people.