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.