Studies show strong reaction to first COVID-19 vaccine may be sign of previous infection

With more than 40 million Americans now fully vaccinated against COVID-19, it's no secret that the shots can lead to unpleasant side effects such as fever, headache, body aches and fatigue. But while initial research indicated that individuals were more likely to experience these symptoms after the second dose, experts now say that those who previously had COVID-19 — whether knowingly or not — may end up reacting more strongly to the first dose.