I’ve written before about animal vaccines against SARS-CoV-2 with regard to mink, but with Russia having recently licensed its Carnivac-Cov vaccine for use in several species (namely dogs, cats, foxes and mink) and Zoetis developing a vaccine in the US for mink, there’s continued interest in the subject, so I’ll revisit it.
I’m glad companies are working on SARS-CoV-2 vaccines for animals. It’s good to be prepared and have the work done in case it’s needed in a particular situation. Whether there’s much use for these vaccines in most animals at this time is hard to say.
When I think about SARS-CoV-2 vaccines for animals, there are three main reasons we would use them:
1. Prevention of severe disease
This is the main goal of most vaccines. However, dogs and cats don’t seem to get very sick, very often, from this virus.
- I’m still on the fence as to whether this virus really causes significant disease in dogs at all. We have some data that suggests infection is associated with risk of very mild disease but overall, it’s still not entirely clear. However, it is clear that they rarely, if ever, get seriously ill.
- Cats are commonly infected with SARS-CoV-2, but even cats rarely develop severe illness. There’s evidence that they can, so we can’t dismiss it. However, our research and others suggests that a large percentage of cats from households where people have COVID-19 get infected. So, we have probably had millions of infected cats worldwide since the start of the pandemic. If this was causing severe disease in a reasonable percentage of cats, I think we’d have clear signs of that by now. Since serious illness can occur in a small minority of cases, vaccination could help, but in the grand scheme of things, given the low risk of severe disease, it’s hard to say that the potential benefits justify the cost and potential adverse events. (We don’t know about any specific adverse effects of vaccination in animals to date. I’m not talking about VITT or the misinformation about mRNA vaccine adverse events in humans. I’m talking about the typical adverse events that we can see in animals with any vaccine.) I’d rather focus on better rabies vaccine coverage and other good preventive medicine things than trying to get cats vaccinated against SARS-CoV-2. I’m not opposed to it. I just think the value is probably limited.
- Mink are different. They are clearly susceptible to infection and it can cause serious illness and even death. The two points below probably are more important when considering vaccination of mink, but there could be mink health benefits from vaccination.
I’d be most interested in vaccination of endangered species with suspected high susceptibility that have human contact, particularly wild felids (e.g. lions, tigers) and non-human primates (e.g. great apes). Vaccination of captive animals (zoos), animals in rehab facilities and animals in areas where there’s close contact with human populations (mainly applicable to primates) might be worthwhile because of the potential implications of infection in endangered populations.
2. Prevention of transmission from animals to people
Dogs pose little risk to people, if any, in terms of transmission of SARS-CoV-2. Dog-to-dog transmission has not been seen experimentally, and infected dogs seem to have pretty low viral loads, so I doubt there’s much risk. Cats pose more risk to people. Cat-to-cat transmission does happen, so we have to assume cat-to-human transmission can occur too. But human-to-cat transmission is still far more common. Because most infected cats probably catch the virus from their owners in the first place, they are probably “dead-end” hosts in households and rarely play a role in transmission. Cats get infected from family members, who have also infected each other, and everyone burns off the virus together. I think the greatest risk of cat-to-human transmission is when infected cats leave the house, particularly to go to a veterinary clinic or shelter. However, the risk associated with that can be significantly reduced using basic infection control practices..
The risk of animal-to-human transmission is probably highest with mink, as mink-to-human transmission clearly occurs. Vaccination of mink could be an important way to control mink-to-human transmission (but preventing human-to-mink transmission is most important).
3. Prevention of viral mutation
This is an important aspect for species that might serve as reservoirs and source of new variants. Virus variants emerge because of random mutations in the viral genome, and mutations happen when the virus replicates. So, the more the virus replicates and the more individuals are infected, the greater chance of a “bad” mutation occurring. For dogs and cats, this isn’t really a big concern. We don’t have massive numbers of dogs or cats together where widespread sustained transmission is of great concern. Yes, a mutation could happen within a single infected dog or cat, but it’s really unlikely (and even then, it’s only relevant if that dog/cat then can pass it on to a person.)
The greatest risk of significant viral mutation in any animal species to date appears to be in mink. We know that the virus can mutate in mink populations AND spread back to people. When you house thousands of a highly susceptible animals close together and introduce the virus from an infected person, that’s the recipe for widespread transmission and massive viral replication that’s needed for “bad”mutations to result in the emergence of a significant variant. Vaccination of mink farm workers helps reduce the risk of mink getting infected and from passing the virus back to people, but there’s still some degree of risk.
Overall, I can’t see a need at this point for vaccination of dogs and cats against SARS-CoV-2. The cost-benefit comparison of vaccination of those species doesn’t seem convincing. Vaccination of high-risk (e.g. endangered) susceptible animals like large cats and non-human primates might make more sense, such as in zoos, rehab facilities and other places where there’s some degree of human contact with these species.
Mink are a different story. If we’re going to continue to farm mink, vaccination is a reasonable consideration.
The final disclaimer here is that all this is based on what we know about current virus variants. New variants always have the potential to reset our knowledge to some degree. Variants that can infect a wider range of animal species, cause more serious disease in animals or be more transmissible from animals to humans might impact the current risks and make vaccination of other species more beneficial. That’s why I’m glad we have some information about vaccines for animals now. I don’t think we need them currently for most species, but it’s good to be prepared should things change.