Live attenuated vaccines reduce impact of COVID-19

Girl receiving live attenuated vaccine to reduce impact of COVID-19

Two studies reported that common childhood vaccines reduced the severity of COVID-19 infection. BCG (bacille Calmette-Guerin) is a vaccine against tuberculosis and the MMR vaccine protects against measles, mumps and rubella. They were both found to reduce the risk of severe COVID-19 symptoms and the need for COVID-19 treatment. You  might ask “Why do vaccines for other ailments have an effect on COVID?” Scientists are finding that live attenuated vaccines often provide broader protection. It is called “trained immunity” where the vaccine kicks the immune system into high gear so it is better able to fight many viruses. For example, children who receive the BCG vaccine at birth have a lower mortality rate from unrelated respiratory infections.

The details of the two studies:

  • A study conducted in Brazil was published in the Journal of Clinical Trials. It was the first double-blind, placebo-controlled, randomized clinical trial that demonstrated action of the MMR vaccine against COVID-19. While the MMR vaccination did not reduce the number of people who became infected with COVID, it greatly reduced the severity of the disease. Participants who took at least one dose of the MMR vaccine had 48% less chance of getting symptoms from COVID-19. And most importantly, the need for treatment for COVID was reduced by 76%. The percentages were slightly higher with two dose of the MMR vaccine. A total of 424 health workers aged 18 to 60 years were enrolled in the study and followed for 5 months to evaluate whether they became positive for COVID-19 and the severity of the infection.  The reasons for the positive impact of MMR vaccine are not totally understood. It may be that the vaccine stimulates our innate immune response, or that some similarities between the MMR viruses and coronavirus.
  • A multi-center, randomised, controlled trial enrolled 495 high-risk participants aged 18–60 years  in India. Some received BCG and some received a placebo. They were followed for 9 months from the date of vaccination. As in the MMR study, there was no difference between groups in the incidence of COVID-19 infection. However, BCG vaccination had a protective efficacy of 62% for symptomatic COVID-19 infection and severity of the disease. Additionally, the effect lasted throughout the study period. The results were published in Infectious Disease and Therapy. (By the way, the BCG vaccine is commonly given in most countries, but is not routinely used in the U.S. and the Netherlands.)

Larger studies may be necessary to confirm these results, but the science behind the generalized protective effect of live attenuated vaccines is becoming clear. Vaccinations such as BCG and MMR can provide simple and cost-effective methods of protecting individuals from severe COVID-19. This may be particularly helpful in low- and middle-income countries without widespread COVID-19 vaccines and treatments available.

For more research on inexpensive COVID-19 treatments, see our COVID-19 page.

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