Measles is an early warning sign for outbreaks of more serious diseases

Many countries give a combined vaccine called MMR, for measles, mumps, rubella. If measles vaccine coverage isn’t high enough, it could in theory be sufficient to prevent major mumps and rubella cases—you need 95 percent of the population vaccinated to guard against measles, but only about 85 to 90 for mumps and rubella. Globally, we’ve held steady at about 85 percent for both MMR and the DTAP vaccine, which protects against diphtheria and also requires 85 percent coverage for herd immunity. (Pertussis and tetanus, the other two viruses in the DTAP shot, can exist in the environment, not just the human body, making herd immunity less relevant).

Should those diseases come back, we may be worse prepared in some ways than before. For a long time, the older generations in our society grew up in the pre-vaccine era, which meant that the overwhelming majority of them were exposed to these viruses. Now we have much less circulation of viruses, but also not sufficiently high vaccine coverage to prevent transmission altogether, and the combination is worrisome. “I’m concerned that there are progressively more countries which have had many years of insufficient vaccine implementation,” Kretsinger says. “It’s hard to predict what will be next.”