Efforts to prevent the spread of COVID-19 also stopped other transmissible diseases, including a polio-like condition called acute flaccid myelitis (AFM), according to a new study. US efforts to control COVID-19 may have had mixed success, but AFM proved easier even without being deliberately targeted.

AFM is a spinal cord infection that reduces peoples' limb strength and control. As with polio, some patients end up with life-long disabilities. After first being detected in the USA in 2012, AFM came back in 2014, 2016, and 2018. The reasons for its two-year cycle are not fully understood, but it wasn't too hard to spot the next number. In August 2020 medical experts warned of another outbreak, likely in the following few months. Instead, only 31 cases were reported, leading to the suggestion protective measures against COVID-19, such as social distancing, made the difference.

A study in Science Translational Medicine links the condition to enterovirus 68 (EV-D68), a respiratory virus that normally causes mild cold-like symptoms in infants and young children. This in turn has confirmed the auspicious actions taken against COVID-19 are the reason there wasn't an AFM outbreak last year.

Although a connection between AFM and EV-D68 has been suspected, EV-D68 has circulated in the US for at least 60 years and is usually harmless enough tracking is haphazard, making a link hard to confirm. Princeton PhD student Sang Woo Park used what information is available on the virus's spread to compare its frequency to that of AFM. Although unable to definitively prove EV-D68 is the cause of AFM, Park and co-authors report a suspiciously strong relationship in when and where they appear. At a minimum, EV-D68 is a good proxy for AFM occurrences.

Polio itself is caused by a (hopefully soon to be eradicated) enterovirus, so it is not surprising similar symptoms would be caused by another member of the same genus of RNA viruses.

The number of people affected by AFM is small compared to polio at its peak, and tiny by comparison with COVID-19. Nevertheless, more than 500 people in the US have been affected by it since 2012, with serious consequences.

"Though currently uncommon, this syndrome has been increasing in frequency with each successive outbreak since 2014, making it critically important to better understand the patterns and drivers behind it," Park said in a statement. 

EV-D68 is more common. Even with limited testing leading to dramatic underreporting, more than 1,000 cases were known in 2014 alone.

The failure of the anticipated 2020 outbreak to occur suggests enterovirus D68 is less transmissible than SARS-CoV-2, making the social distancing and mask use that only partially controlled the COVID virus more effective against AFM. We've seen a similar plunge in influenza, which is known to be less transmissible than SARS-CoV-2.

As vaccinations take over from social distancing as the prime method of COVID-19 control, Park thinks EV-D68 will likely return, bringing AFM with it. A vaccine against it may prove essential.