Concerns about the animal aspects of the COVID-19 pandemic continue to come in waves. Most of the time they are ignored or dismissed, but there are also periodic flurries of attention and (often over-) reaction. Lately, questions about vaccination of animals against SARS-CoV-2 follow have been on the rise.
Should domestic and wild animals be vaccinated against SARS-CoV-2?
- Yes, no and maybe… but mainly no. To properly assess this question, we need to step back and think about what vaccines can potentially do.
There are 4 main areas I think about when considering whether vaccination may be useful in any given species:
- Prevention of disease:
- This is really “prevention of severe disease.” If a species doesn’t get very sick and just has mild, transient illness, there’s little value in vaccinating from an animal health standpoint.
- Prevention of transmission of the disease to people:
- Transmission of SARS-CoV-2 from animals to humans has been documented or suspected in very few species. Most transmission is human-to-animal, and animals are most often dead end hosts (e.g. if my cat gets infected, he got it from someone in my household and is unlikely to spread the virus to any other humans).
- Prevention of transmission to other animals:
- Can the animal spread SARS-CoV-2 to other animals of the same species? Or be a bridge, spreading the virus to other domestic animals or wildlife?
- Prevention of establishment of an animal reservoir where new variants could emerge:
- This is the big concern with certain animal species (e.g. deer, mink) that are both highly susceptible and have a large population with frequent, close contact between animals, which allows for for potential long term, continued transmission of SARS-CoV-2.
Not many species check many (or any) of those boxes when it comes to vaccination against SARS-CoV-2.
The other thing to consider is what vaccines for animals may or may not actually be able to do. We have very little information about the very limited vaccines for animals that are currently out there. These vaccines are based on older technology than our human mRNA vaccines, and the older tech hasn’t been known to be able to produce highly effective coronavirus vaccines in the past. Vaccine science is improving, and I’m not trying to bash the animal vaccines or the companies (I’m grateful they’re working on them), but I’m trying to be realistic.
Vaccines can have a range of effects, depending on their efficacy:
- Reduction of severe disease
- Reduction of disease
- Reduction of infection (with or without disease)
- Prevention of infection (sterilizing immunity)
We don’t know how effective animal vaccines are in different species. Based on what we know about the more technologically advanced human vaccines, reduction of severe disease is likely a much more realistic target than reduction or prevention of infection (which would reduce the risk of transmission).
Another issue is the level of vaccine coverage that would be needed in a given species. For example, if we want to use vaccination as a tool to reduce establishment of the virus in wildlife, we need a vaccine that significantly reduces infection and transmission AND we need to vaccinate a large percentage of the population AND keep vaccinating animals over time, since population turnover is high in many wildlife species.
We also don’t know how long immunity persists in different species, or how it might hold up against different variants of SARS-CoV-2 (which we wonder about it humans as well…).
Once we start thinking about all these factors, I think it shows that the utility of vaccination in animals currently is going to be limited to a few niche situations.
Dogs / cats
- Dogs and cats commonly get infected, but rarely get seriously ill. They are rarely going to be the source of infection for people (they are usually infected by their owners).
- Dogs and cats don’t live in large groups where virus transmission can be sustained long term, such that it could create a reservoir and become a source of virus variants.
- Indoor-outdoor cats are more of a risk as a bridge between infected households and other people or animals, but the odds of them playing a significant role in transmission are fairly low AND any effect on this would require a vaccine that prevents infection and virus shedding (unlikely). The far easier and more effective solution is to keep cats from infected households indoors until the risk period for transmission has passed.
- I’m glad we have vaccines ready in case something changes and we get a strain that causes more serious disease in dogs or cats, but I can’t see a use for vaccination in these species now.
- Mink check a few of the boxes for vaccination, as a species that is susceptible, gets sick, is housed in large populations (i.e. farmed mink), that has generated new variants in the past, and can spread the virus to people and other species.
- Vaccination of all mink on a farm is also possible.
- Vaccination is probably a more effective tool for mink health than public health, given the questions above about whether vaccines actually reduce or prevent infection, and therefore transmission. If vaccination just reduces disease but still allows for transmission between and from mink, then it might not be useful or might even be counterproductive for public health purposes.
- Here’s where there’s the most potential for benefits from vaccination against SARS-CoV-2 in animals. Some zoo species are highly susceptible and can die from infection with this virus. These animals can be valuable emotionally and economically, and from a conservation standpoint. So, more zoos are vaccinating their non-human primates (e.g. apes) and big cats. Some are also vaccinating their mustelids (i.e. species related to mink; some, like the black footed ferret, are highly endangered) and cervids (i.e. species related to deer, since we know white-tailed deer are quite susceptible and can spread the virus).
Wild deer (specifically white-tailed deer)
- Deer check a few of the boxes for vaccination, as they are highly susceptible and may be able to maintain circulation of the virus in area where they have a high population density, potentially leading to new “deer” variants.
- However, we would need a vaccine that significantly reduces infection and therefore the risk transmission (and I’m not very confident we have that). Deer don’t appear to get significantly ill themselves (thus far), so vaccination isn’t directly helpful to the deer.
- To be useful, it would also require vaccination of a LOT of animals. For example, if we need to vaccinate 80% of the wild deer population… well, good luck finding a way to do that.
- Furthermore, lots of new deer are born every year so vaccination campaigns would have to be continued until SARS-CoV-2 is out of circulation in deer and people. That doesn’t seem very practical to me. Rabies vaccination of some wildlife species is used in some areas (including Ontario) and it’s highly effective. However, it uses a highly effective vaccine that can be delivered through baits that can eventually cover a pretty high percentage of the target population, and it does have to be repeated at least annually until the virus is eradicated from an area. It also takes a lot of effort and coordination. It would be very tough to do the same for SARS-CoV-2 vaccination in deer.
We’re not going to vaccinate our way out of animal issues with this virus. We need to control SARS-CoV-2 in humans to have any hope of controlling SARS-CoV-2 in animals. Vaccine research is important so that we have vaccines available should opportunities for their use be identified, and continued vaccine development may get us to the point where we have a highly effective vaccine that stops transmission. However, without that, the potential impact of vaccination of most animals is limited.