Routine vaccines have prevented the deaths of about 154 million children around the world over the past 50 years, a new study shows, but efforts have been slowing recently, allowing for the growth of some vaccine-preventable diseases. This backslide could lead to many more unnecessary illnesses and deaths without an increased effort to vaccinate children and counter misinformation.

The report, published Tuesday in the medical journal The Lancet, says that over the past five decades, the World Health Organization’s Expanded Programme on Immunization has vaccinated more than 4 billion children. This doubling of global coverage of vaccines has prevented countless cases of tuberculosis, measles, polio, diphtheria, tetanus and pertussis.

There was a concerted effort to vaccinate people between 1980 and 2023, the researchers found, resulting in a 75% drop in the number of children who are “zero-dose” — those younger than 1 who have not received any doses of the diphtheria-tetanus-pertussis vaccine, which is often used as a way to measure overall immunization coverage.

“Childhood immunization has proven to be one of the most successful and cost-effective public health strategies known both in terms of lives saved and return on investment,” the researchers wrote. The financial rate of return is, in some instances, up to 44 times the cost of vaccination, they say.

But despite this success, the study says, vaccination efforts have been slowing, and in some cases, progress has even been reversed.