Late-stage studies of AstraZeneca's COVID-19 vaccine candidate are on temporary hold while the company investigates whether a patient had a serious side effect linked to the shot. In a statement issued Tuesday evening, the company said it was pausing vaccinations to look into “a potentially unexplained illness," to see if the link is real or a coincidence.

Temporary halts in medical studies aren't uncommon.

Two other vaccines are in huge, final-stage tests in the United States. One is made by Moderna and the other by Pfizer and Germany’s BioNTech.