Measles is an extremely contagious disease that used to be one of the most prevalent human diseases in the world. For centuries, mankind struggled to make any progress against measles, until vaccines came along.

The measles vaccine, which was first introduced in the 1960s, turned out to be extremely effective against the disease. Modern vaccines offer even more protection than the original ones, having an efficacy of 93% of one dose and 97% for two doses.

But vaccines are only effective if you take them.

In 2020, while the world was mostly focused on the pandemic, 22 million babies missed their measles vaccines, 3 million more than in 2019 — the largest increase in over 20 years. This threatens to reverse the progress that’s been painstakingly made over the past few years.

To make matters even worse, there’s a good chance that official counts are underestimating the real number of measles cases. News reports in 2020 of localized outbreaks suggest the disease may be spreading unnoticed where vaccination is lacking.

“Even before the pandemic, we were seeing how even small pockets of low measles immunization coverage could fuel unprecedented outbreaks, including in countries where the disease had been considered eradicated. And now, COVID-19 is creating widening gaps in coverage at a pace we haven’t seen in decades,” Ephrem Tekle Lemango, UNICEF’s associate director for immunization, said in a statement.

“While we have not seen an increase in cases yet, measles is simply too contagious. If we do not act, gaps will become outbreaks, and many children will be exposed to a preventable but potentially deadly disease,” he added.

This is why, although officially there hasn’t been much of a spike in measles cases, officials warn that it’s only a matter of time before this starts happening. Measles is simply too contagious of a disease to allow it any pockets to spread in — if such pockets exist, the virus will soon find its way to them. Already, some places are starting to show the occasional small but significant outbreak.

It’s estimated that worldwide, measles vaccinations prevent more than 31 million deaths a year, mostly in children. This is why, the CDC urges that, even during the COVID-19 pandemic, it is vital that measles vaccination campaigns continue unabated.

“Large numbers of unvaccinated children, outbreaks of measles, and disease detection and diagnostics diverted to support COVID-19 responses are factors that increase the likelihood of measles-related deaths and serious complications in children,” Dr. Kevin Cain, CDC’s global immunization director, said in a statement. “We must act now to strengthen disease surveillance systems and close immunity gaps, before travel and trade return to pre-pandemic levels, to prevent deadly measles outbreaks and mitigate the risk of other vaccine-preventable diseases.”

If there’s anything we’ve learned from this pandemic is that you can’t ever let your guard down against an infectious virus — the moment you do it, it can pop up and cause trouble.