Science Matters - Is Covid here to stay?

By Dorothy H Crawford

 

As a virologist, I have avoided writing about Covid in this column only because there has been quite enough in the media to satisfy everyone’s curiosity.

 

But now we are facing the issue of whether or not the virus – SARS-CoV-2 - can be eliminated globally.

 

The Chinese government, with its zero tolerance policy, clearly thinks it can, but others suggest that learning to live with the virus is our best option.

 

Smallpox is the only human virus ever eliminated, finally extinguished in 1980.

 

The success of this worldwide eradication campaign was based on four facts: that there was a single, stable smallpox virus strain; the virus only targeted humans; there was an effective smallpox vaccine; and all infections produced overt disease.

 

But none of these essential criteria apply to SARS-CoV-2.  For this virus round 20 per cent of infections are silent and its high mutation rate could cause any vaccine to become less effective over time.

 

An un-identified wild, virus-carrying animal species presumably initiated the first infection in China in 2019, and since then spill-over from humans to several other susceptible animals species has occurred.

 

SARS-CoV-2 quickly jumped to several domestic and zoo animals, and in November 2020 spill-over from humans to farmed mink in Denmark initiated spillback of a mutated virus from mink to humans, necessitating the culling of 17 million mink (1).

 

More recent spillback events include pet-shop hamsters initiating a Covid outbreak in Hong Kong (2), and similarly a sneezing cat infecting a vet in Thailand (3).

 

While these spillback events are rare, scientists have now discovered widespread SARS-CoV-2 infection in white-tailed deer in the US and Canada.

 

The flurry of investigations that followed this discovery indicates that the virus has jumped from humans to deer on at least six occasions, that around 40 per cent of white-tailed deer were virus antibody-positive in one study (4), but deer infection does not cause disease.

 

There is still much to uncover here, crucially, whether the virus establishes a long-term reservoir in white-tailed deer that could spillback into humans and on to other wildlife, perhaps in mutated forms. If so this would certainly stymie global elimination of the virus any time soon.

 

1) BBO Munnink et al. Science 371, p172-77. 2021. 2) H-L Yen et al. Lancet 399, p1070-78. 2022. 3) T Sila et al. Emerg. Infect. Dis. 28, 1485-88. 2022. 4) JC Chandler et al. PNAS Sci USA 119, 2022.