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. 

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