A new pandemic

These days I commonly get the question ”Does vaccination of dogs with available canine influenza vaccines protect them from H5N1 avian flu?” While we don’t have any hard data on this, we still have a pretty good idea of the answer.

One of the challenges with flu vaccination (in any species, including humans) is the lack of good cross protection between strains (e.g. H3N2 vs H1N1 vs H5N1). In general, we assume there’s little to no cross-protection, and organizations such as CDC, Public Health Agency of Canada and WHO have stated that seasonal flu vaccines don’t offer any protection to people against H5N1 flu. 

That said, theoretically there there may be a bit of protection, though it’s hard to have confidence in that based on data from older lab animal studies. One study (Rockman et al. J Virol 2013) showed that ferrets vaccinated with human seasonal flu vaccine had partial protection against challenge with H5N1 flu. 

They determined that this was from the H1N1 component of the vaccine, and was predominantly from the neuraminidase (N) part (H5N1 and H1N1 have the same N1). 

While we typically pay more attention to the H component for vaccination, the similarity in N may be useful. That means there may be some protection of people from H5N1 flu if they’re vaccinated for seasonal flu, but not a lot, and we have to be careful extrapolating too much from older studies using different types of H5N1 in lab animal models. 

Canine flu vaccines target H3N2 canine flu, plus or minus H3N8 canine flu strain. (Realistically we only care about H3N2 now, since H3N8 appears to have disappeared as of a few years ago.) Those are quite different from H5N1 avian flu in flu terms.  Since there’s no overlap in the Hs or Ns, we wouldn’t expect to even have that small theoretical cross-protection benefit. If we had an H1N1 canine flu strain, maybe there’d be some protection, but (thankfully!) we don’t.

Does canine flu vaccination help protect against flu recombination? 

Recombination (mixing) of different flu strains in the same host (human or animal) to create a new, more problematic strain is certainly a concern. We don’t want someone to be infected with seasonal flu and H5N1 flu at the same time, as that creates the potential for a new flu strain to emerge that has the hallmarks of a human flu (readily infected people, effective human-to-human transmission) but has picked up enough H5N1 bits that we don’t have protection from previous exposure or vaccination and may can cause more severe disease. So even though seasonal flu vaccines in people don’t protect against H5N1 flu, there is still benefit from reducing the human seasonal flu burden, as it in turn reduces the risk that a person will be infected with multiple flu strains at the same time, which reduces the chances of recombination.

Does the same principle apply to vaccination of dogs against canine flu? It’s a stretch. There could be some potential benefits for canine health, but probably not much benefit for public health. Canine flu is much rarer than seasonal flu is in people, so there’s less potential benefit simply because there’s less disease to prevent. There’s also less baseline protection against canine flu in dogs because it’s rare and therefore vaccination is uncommon. So from a disease transmission standpoint for dogs, a new flu strain against which they don’t have immunity isn’t that different from the current H3N2 canine flu strain, since most dogs don’t have immunity to that either.