Endless news cycles and viral social media warn of “breakthrough infections” in people already vaccinated for COVID-19. These reports leave the mistaken impression that protections afforded by the vaccines are not working—and they can fuel reticence among the millions of people in the U.S. who have yet to get a shot. But such infections are not only known to occur after COVID vaccination. They frequently happen following inoculation against influenza, measles and many other diseases.
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A “breakthrough” simply means that a vaccinated person has tested positive for the disease-causing agent, not that they will become ill or transmit the infection to someone else. Most vaccinated people who are infected do not have symptoms, and those that do tend to have mild illness. Even with the Delta variant of SARS-CoV-2, the vaccines show good protection against symptomatic disease and death.
Breakthrough cases do not occur because the vaccines are ineffective. Immunity can wane over time, and a vaccine might be less effective for a given pathogen. The measles-mumps-rubella (MMR) vaccine is one example: its protection against measles is strong, but the immunity to mumps it confers is less so, [associate professor at Johns Hopkins Bloomberg School of Public Health Kawsar] Talaat says.
