Countries are beginning to offer a fourth dose of the Covid-19 vaccine to vulnerable groups, but medical professionals are undecided on whether it would benefit the wider population.
The U.S. Food and Drug Administration has so far authorized a fourth shot only for those aged 50 and above, as well as those who are immunocompromised. And the U.S. Centers for Disease Control and Prevention was skeptical of the need for a fourth dose for healthy adults in the absence of a clearer public health strategy.
Those decisions came as a study from Israel found that although a fourth dose of the Pfizer-BioNTech vaccine offers protection against serious illness for at least six weeks after the shot, it provides only short-lived protection against infection, which wanes after just four weeks.