Measles once devastated the world like COVID. It still threatens us today.

It comes as no surprise — to epidemiologists certainly — that as measles vaccinations around the world have decreased over the past few years, measles outbreaks have increased. And they have increased everywhere, from Idaho in the U.S. to Pakistan to cities in Kyrgyzstan to two of the largest cities in Australia. The places that have been the most hesitant about immunizing children have also been the most at risk.

Measles deaths spiked 43 percent in 2022, according to a recent report from the World Health Organization and the U.S. Centers for Disease Control, but this does not look like it will be only a one-year phenomenon.

Take Pakistan, for example, where at least 41 healthcare workers engaged in immunization campaigns have been killed since 2016. In the second and third quarters of 2023, more than 9,000 cases of measles have been reported, and the country’s immunization rates for the first dose of measles vaccine are hovering just above 80 percent, well below what is needed to stop the disease. In Idaho, where a smaller measles outbreak has recently emerged, children in kindergarten have almost the same immunization rate — slightly more than 80 percent.

Measles is a highly contagious viral infection that causes fever and a rash. The disease occurs most commonly in young children but can occur in susceptible older children and adults too. Most children with measles recover but the disease can be fatal — pneumonia remains the biggest cause of death in children with measles.

Before the widespread use of measles vaccines, measles resulted in several million deaths each year— roughly the equivalent of the first year of the COVID-19 pandemic. After the measles vaccine was developed and immunization campaigns were launched around the world, starting almost 50 years ago with the Expanded Program on Immunization, this number was dramatically reduced. But measles is still estimated to kill 100,00 to 200,000 children annually, an unacceptable estimate as all of these deaths are preventable.

Two doses of measles vaccine provide nearly complete protection against disease. Measles vaccines are low-cost, very safe and do not cause autism. The problem is that not all children are vaccinated.

What makes measles outbreaks particularly difficult to control is that the infection is so highly contagious. The scale that we use to measure how “spreadable” infections are is called the basic reproductive number; this scale became well-known during the COVID-19 pandemic. It is the average number of people who would be expected to get infected if a single infectious individual enters a completely susceptible population, where no one was vaccinated or had the disease.

For the original variant of SARS-CoV-2, the virus that causes COVID-19, the basic reproductive number was about 3. For the later omicron variant, this number increased to as high as 9. But for measles, the basic reproductive number often ranges from 12 to 18.

The only method we have found to stop virus transmission and prevent measles outbreaks is a very high level of population immunity. In many settings, more than 90 to 95 percent of people in a community need to be immune to prevent measles virus spread. This is only achieved when almost everyone has received two doses of measles vaccine. 

Yet before the COVID-19 pandemic, despite enormous global efforts, too many of the world’s children did not receive even a single dose of the measles vaccine.

This situation worsened during the COVID-19 pandemic, which had a big impact on the delivery of essential health services — including childhood immunizations. According to the World Health Organization and UNICEF, the proportion of children who received one dose of measles vaccine in 2019 was 86 percent. But this proportion dropped to 81 percent in 2021, rebounding to only 83 percent in 2022. 

These changes may seem slight but represent an increase of 2.7 million unvaccinated children, from 19.2 million in 2019 to 21.9 million in 2022. We need to do more than catch up to our numbers from three years ago, we need to redouble our efforts so that this number is drastically reduced.

The one saving grace that we have had so far is that the preventive measures put in place to prevent COVID-19, such as masking, social distancing and restricted travel, helped limit the spread of the measles virus among this increased number of vulnerable children. But those preventive measures are no longer in place.

Reported measles cases were at historic lows during the pandemic. But beginning in 2022 and continuing today, measles cases are increasing. We have seen particularly large outbreaks reported in Pakistan and many other places: India, Yemen, Cameroon, Nigeria and the European Union. In the U.S., an outbreak in Ohio last winter was concentrated among unvaccinated children, with more than a third of infected patients needing hospitalization.

Given the increase in susceptible children who missed out on measles vaccination, the number and frequency of these measles outbreaks will grow larger. And because these outbreaks are preventable, we need to act now and act fast. We need to strengthen our primary health care systems to deliver immunizations to all eligible children, making special efforts to reach those who missed out on routine vaccinations and are most vulnerable. And we need to do this everywhere. 

All children are precious; it’s time to stop leaving them vulnerable to a disease we can easily prevent.

William Moss is the executive director of the International Vaccine Access Center at Johns Hopkins Bloomberg School of Public Health.