The MMR vaccine could be partly to thank for children's low rate of COVID-19 infections and better prospects for survival.
Researchers in the US have found an inverse correlation between antibodies generated by the vaccine - which protects against measles, mumps and rubella - and the severity of COVID-19 infections in people aged under 42.
"The MMR vaccine may be protective against COVID-19," said study lead author Jeffrey E Gold, president of Georgia-based charity World Organization.
"It also may explain why children have a much lower COVID-19 case rate than adults, as well as a much lower death rate. The majority of children get their first MMR vaccination around 12 to 15 months of age and a second one from four to six years of age."
The MMR vaccine was first introduced in the early 1970s, and has few side-effects. It has been used in New Zealand since 1990.
"This is the first immunological study to evaluate the relationship between the MMR II vaccine and COVID-19," said co-author David J Hurley, professor and molecular microbiologist at the University of Georgia.
The study involved two groups of subjects who'd contracted the coronavirus - 50 who'd had the MMR vaccine, and 30 who hadn't.
Of those who'd had the vaccine, the more mumps antibodies they had, the less severe their infection was. For those who hadn't had the vaccine, but still might have had antibodies from a prior infection, there was no correlation at all.
"The MMR II vaccine is considered a safe vaccine with very few side effects," said Dr Hurley.
"If it has the ultimate benefit of preventing infection from COVID-19, preventing the spread of COVID-19, reducing the severity of it, or a combination of any or all of those, it is a very high-reward, low-risk ratio intervention."
He said even people over 40 might benefit from getting the MMR vaccine.
A number of COVID-specific vaccines have shown great promise in recent weeks, but their long-term efficacy and safety remains unknown.
A possible flaw in the research is that the average age of the group with the MMR vaccine was 30, and those without 57. The researchers said there was no way to get around this, as most people with the vaccine are young - it hasn't been around forever.
They said the rapid spread of COVID-19 across the US and Europe could be linked to "problematic" rates of vaccination in the US and Europe, thanks to the anti-vaccination movement.
"While travel restrictions, control of congregate living conditions, and governmental interventions play roles in controlling the COVID-19 spread, it is possible that MMR vaccination programs are the basis for the huge international variation," the study - published in mBio, published by the American Society for Microbiology - read.
