Mink are back in the news, mainly with respect to vaccination against SARS-CoV-2. Mink are very susceptible to this virus, and it’s been shown that they can transmit it back to people. Perhaps more of a concern is that several mutant strains of the virus have also emerged in mink, though it’s not really surprising, since more transmission (especially with a species jump) means more risk of mutation.
The question of vaccinating mink against SARS-CoV-2 keeps coming up, and efforts to develop vaccines are underway. I’m no vaccinologist, so keep that in mind as you read this.
Vaccines can be used for a number of reasons, but I’ll focus on two broad categories:
- Reduce/eliminate disease
- Reduce/eliminate infection
There are important differences between these purposes. Ideally we want a “sterilizing” vaccine, which prevents infection altogether. That means when the virus encounters a vaccinated individual, it doesn’t do anything – it can’t replicate enough to establish an infection.
Unfortunately, most vaccines aren’t that good. Most vaccines can reduce the likelihood of disease or severity of disease, but the virus can still commonly infect vaccinated individuals to some degree. That can still be useful, as the primary goal is usually to reduce illness. However, if those infected but somewhat protected individuals can still transmit the virus to others, it’s not as good for disease control overall.
But even if a vaccine isn’t great, it can’t hurt, right?
- That’s not clear. There are a few potential concerns with vaccinating mink with a non-sterilizing vaccine.
Maintaining a reservoir
If a vaccine just reduces disease but not infection, that’s good for the individual mink, but could be bad for people. It would mean the mink are still susceptible to infection (which is typically introduced to the herd by infected people). If the virus then spreads widely and silently on the farm because the mink aren’t getting sick, it’s harder to detect and control. That means farms may be more likely to become silent reservoirs of the virus.
More risk of mutations?
Virus mutations are random events, but the more a virus spreads and replicates, the greater the risk that these random events can occur. If a mutation results in increased transmissibility, increased virulence or poorer vaccine effectiveness, and that strain spread back into people, that’s obviously bad. If the virus is circulating silently on a farm, it is likely to do so for longer before it’s detected and brought under control, providing more opportunity for mutant strains to emerg.
Encouraging vaccine-resistant mutants
A vaccine that’s only marginally effective might actually help select for vaccine-resistant mutants of the virus. The big concern with that is if those mink vaccine-resistant mutants are also resistant to human vaccines, and then they spread to people, then that strain could spread even within the vaccinated human population. We don’t know if this is an issue, but it’s been raised in the context of people with suboptimal immunity to SARS-CoV-2 after only receiving a single vaccine dose, while awaiting their second dose.
I’m not saying don’t vaccinate mink. It might be a useful control tool (for mink and people).
- Don’t think about vaccination as the main control measure for SARS-CoV-2 infection in mink. It can’t be done in lieu of other infection control practices.
- Consider (and investigate) potential unintended consequences of vaccination (and other control measures).
- Don’t rush to market a crappy vaccine on basis of “it can’t hurt,” because maybe it can.
If it’s going to be done, we need to make sure it’s done right. Cheap, rushed, suboptimal vaccines (in humans or animals) might make things worse. There are enough examples of pretty useless vaccines for animals that are on the market, so it’s a realistic issue. Conditional licencing of animal vaccines usually doesn’t require much data beyond indicating it’s unlikely to be harmful in the individual, but doesn’t account for potential effects at the population level.
The best way to prevent issues in mink is to reduce circulation of the virus in people, so the mink don’t get exposed through infected people in the first place. Keeping people away from mink and using better infection control practices are also important (or having fewer captive mink).