Are Second Covid Booster Shots Effective? What Experts Know So Far

Everyone agrees that more coronavirus variants are likely. But how much the virus will evolve and how long existing vaccines will continue protecting against severe cases of Covid-19 remains uncertain. That’s led a short list of countries to recommend second boosters of existing vaccines for the especially vulnerable. These doses -- often referred to as a fourth shot, though it will be the third for those who initially got the single-dose immunization made by Johnson & Johnson -- are essentially a stopgap measure. Longer term, many researchers believe the vaccines will need to be periodically updated to counteract new strains, just as flu shots are tweaked annually.

It mostly comes from Israel, the first country to offer them. The rate of severe Covid among people who received a fourth dose of Pfizer’s vaccine was about three times lower than those who got only three shots, according to an Israeli study published April 5 that analyzes data from some 1.2 million people. The study, which focused on people 60 and older, also found that those who received three doses reported twice as many infections as those with the additional booster, although this benefit appeared to be short-lived. A big caveat, according to Sam Fazeli, a senior pharmaceutical analyst at Bloomberg Intelligence, is that behavioral or demographic differences may play a role in the findings. For example, those who opted for a fourth dose could have had a lower risk appetite than those who had only three doses. A second study from Israel looked at fourth shots of Moderna and Pfizer vaccines in health-care workers with an average age in the 50s. It found the fourth shots boosted antibodies but had only modest impacts in preventing infections.