A new coronavirus strain has appeared on Danish mink farms. Since June mink-related variants have infected more than 200 people, about a dozen of whom had a mutation called “Cluster 5.” In order to prevent further spread, on November 4 the Danish government announced that all mink in the country would be killed, although the plan was subsequently put on hold amid opposition from lawmakers. The fear is that this mutation has the potential to decrease the efficacy of COVID-19 vaccines currently under development. Information about the mutations published to date does not support this claim. Nevertheless, it remains unclear what effect these mutations might have.
Like ferrets and several other animals, mink are susceptible to SARS-CoV-2 and capable of spreading it to members of their own species. In addition, they can also pass the virus on to humans, as several cases in the Netherlands this past June demonstrated.
The Danish coronavirus variant exhibits two mutations that, according to the scant information available thus far from the Statens Serum Institute in Copenhagen, affect the spike protein that the virus uses to infect cells. According to the institute’s analyses, the mutations decrease the efficacy of antibodies. Researchers discovered in August that one of the mutations deactivates antibodies in the therapeutic cocktail made by the company Regeneron, which was used to treat U.S. president Donald Trump.
Although some initial conclusions can be drawn about the fundamental characteristics of the novel coronavirus from laboratory studies and computer analyses, they do not tell us how the virus actually behaves in humans.
SARS-CoV-2 Already Binds Very Well
The challenge of understanding SARS-CoV-2’s behavior became apparent during discussions around a previously identified mutation called D614G, which characterizes a dominant virus strain found especially in Europe and along the East Coast of the U.S. It is still unclear whether this strain’s elevated transmissibility observed in the laboratory also occurs in the cells of living people.
This also holds true for the two main mutations, which, according to a team led by Jesse Bloom of the Fred Hutchinson Cancer Research Center, increase how well the spike protein binds to human ACE2 receptors. As Bloom’s lab tweeted in early November, “What does this mean for human transmission or disease? Impossible to say, but probably nothing at least for transmission.” Bloom says there is no indication that SARS-CoV-2 is evolving toward increased binding ability. The virus’s existing machinery presumably works sufficiently well.
Other experts in the field also doubt whether the mutations found in minks will prove directly relevant to the pandemic. Geneticist Francois Balloux of University College London tweeted that the mutations are nothing to worry about. Because of their already high mutation rate, variants of this sort and others have probably already appeared in humans. If they actually provided a significant advantage in terms of transmission, they would have become more widespread long ago. Presently, the World Health Organization (WHO) also sees no evidence of elevated risk. According to WHO chief scientist Soumya Swaminathan, speaking from Geneva on Friday, there have already been numerous SARS-CoV-2 mutations. “It is too early to jump to conclusions as to the implications of what these specific mutations have” for transmissibility, severity of disease, clinical symptoms, immune response or potential vaccine efficacy, she says.
The situation may be different for “escape mutations,” which enable the virus to evade an immune response—and even to compromise the efficacy of the vaccine. According to Bloom, it is possible for a mutation to have a small antigen effect, meaning it affects binding of the antibodies produced by the vaccine. But based on his own research, he does not believe that such a mutation alone would dramatically decrease the effectiveness of the immune response.
Finding Refuge in Animals
Mutations are not the only problem raised by SARS-CoV-2-infected mink, however. The larger issue is the potential for the virus to spread independently in animal populations. Researchers fear that mink and related species will form a “reservoir,” providing a permanent refuge for the virus. That would significantly complicate the battle against SARS-CoV-2. For example, say the virus has been completely eradicated in one region. If, however, it continues to be present in domesticated animals or even pets, it could reemerge at any time as if out of the blue.
Even if there is a sufficient supply of vaccine, there will not be enough of it available at first to vaccinate the entire population—or even high-risk groups. So the supply that is available must be used in a targeted campaign to contain the virus. “Ring vaccination,” in which a sort of protective wall is created around people known to be infected by vaccinating their contacts and their contacts’ contacts so that the virus is unable to spread further, could be an important approach.
That would be a promising strategy if coronavirus cases decrease significantly next summer, especially because only a small percentage of people infected actually infect others. This observation explains why many infection chains simply peter out on their own. Even without a vaccine, some countries have been able to virtually eradicate the virus using targeted measures alone. With a vaccine, the chances of halting the disease would increase significantly elsewhere as well.
The situation changes, however, as soon as the virus finds refuge and establishes itself in animal hosts. At that point, breaking the chain of human infection is no longer enough because animals can cause new outbreaks. This scenario is not at all implausible because, aside from mink, some other species may have a high susceptibility to the virus. And some of these animals may be preyed upon by cats, which also are vulnerable to SARS-CoV-2. To date, cats have played no role in the pandemic, but it is unclear what might happen if they come into regular contact with infected wild animals.
This article originally appeared in Spektrum der Wissenschaft and was reproduced with permission.
Editor’s Note (10/11/20): Spektrum der Wissenchaft has updated this story after posting to clarify the number of people infected with a new mink coronavirus variant.