A growing number of countries are looking at switching to different Covid-19 vaccines for second doses or booster shots amid supply delays and safety concerns that have slowed their vaccination campaigns.

Several medical studies to test the efficacy of switching Covid-19 vaccines are underway. A mixed schedule, where a shot of Pfizer's vaccine is given after an AstraZeneca shot, produced more antibodies than two AstraZeneca shots, an Oxford study said on June 28...