The MMR (measles, mumps, rubella) vaccine could help to prevent inflammation in COVID-19, which is associated with the most severe symptoms of the disease.
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There is growing evidence to suggest that using existing vaccines could be beneficial against COVID-19 — even though they are not specific to the novel coronavirus SARS-CoV-2.
Several clinical trials are taking place around the world to test whether using the BCG (Bacillus Calmette–Guérin) vaccine, which protects against tuberculosis (TB), could be effective in COVID-19.
Scientists think that the vaccine could boost a person’s immune response, reduce their levels of SARS-CoV-2, and lessen the symptoms associated with COVID-19.
A new report published in the American Society for Microbiology’s journal mBio suggests that the MMR vaccine, which is routinely administered in childhood, could serve a similar purpose.
The researchers behind the article suggest that the vaccine could dampen the severe inflammation associated with COVID-19 and mortality, and propose a clinical trial for healthcare workers.
Training the immune system
The type of vaccines that have an effect against unrelated infectious are called live attenuated vaccines. This means they contain real viruses or bacteria that scientists have weakened in a laboratory.
Studies show that these vaccines protect against other infections by ‘training’ the immune system in a non-specific way. This type of non-specific immune response is the first line of defense against infection and is called the innate immune response.
“Live attenuated vaccines seemingly have some nonspecific benefits as well as immunity to the target pathogen,” explains co-author of the new paper Dr. Paul Fidel, Jr., Associate Dean for Research at Louisiana State University Health School of Dentistry in New Orleans.
Dr. Fidel, together with Dr. Mairi Noverr, Professor of Microbiology & Immunology at Tulane University School of Medicine in New Orleans, suggest that the protection afforded by these vaccines is due to myeloid-derived suppressor cells, or MDSCs, a type of immune cell that comes from bone marrow.
Scientists have shown that these cells can reduce inflammation and mortality in mouse models of infection. In their own research, Drs. Fidel and Noverr have shown that vaccination with a live-attenuated fungus may protect against sepsis thanks to MDSCs.
The evidence behind the theory
Drs. Fidel and Noverr say an MMR vaccine could induce MDSCs in people with COVID-19, which could help them to fight the lung inflammation and sepsis associated with the most severe forms of the disease.
As evidence for this, they cite the recent case where 955 sailors on the U.S.S. Roosevelt tested positive for COVID-19 but only experienced mild symptoms.
The researchers suggest that this may be because all U.S. Navy recruits receive the MMR vaccination. However, it is likely that the age and fitness of the sailors also played a part in their recovery.
Other evidence in support of a connection between the MMR vaccine and COVID-19 recovery comes from epidemiological data. People who live in areas that routinely receive the MMR vaccine have lower COVID-19 death rates.
COVID-19 is also less likely to affect children, which may also have links to vaccination. While age is a known risk factor for COVID-19 severity, the fact that many children have had more recent exposure to live attenuated vaccines could also play a role in their protection, the article suggests.
Progressing to clinical trial
Adults who received the MMR vaccine as a child will likely still have antibodies against the measles, mumps and rubella viruses, but are unlikely to still have MDSCs, says Dr. Fidel. This means they would require a ‘booster’ vaccination to obtain the potential benefits against COVID-19.
“While the MDSCs are long-lived, they are not life-long cells. So, a booster MMR would enhance the antibodies to measles, mumps, and rubella and reinitiate the MDSCs. We would hope that the MDSCs induced by the MMR would have a fairly good life-span to get through the critical time of the pandemic.”
The researchers have proposed a clinical trial of the MMR vaccine in high-risk healthcare workers and first responders in New Orleans. They have also been awarded a grant to compare the MMR and BCG vaccines in a primate model of COVID-19.
If their hypothesis is correct, the authors say use of the MMR vaccine could present a ‘low-risk-high-reward’ measure to save lives during the COVID-19 pandemic.