Can vaccinated people still spread COVID-19? How long does immunity last? Here’s what science knows now. – The Philadelphia Inquirer

But those promising results came from rigorous clinical trials, in which participants who met specific criteria were assigned at random to receive either the vaccine or a placebo, allowing their outcomes to be compared. All sorts of people were excluded: pregnant women, people who had previously been infected with COVID-19, and patients with certain cancers, HIV, or severe allergies.

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