In my last post I used a photo of an old (positive) test result for COVID (this is also an old photo of my sweet baby, hence the grainy appearance) and I noticed it included Influenza A and B as possible results as well, which got me thinking about the flu and wondering how much people know what the flu even is. The question also ties into vaccines and how experts decide on which “strain” to use – and do people even know what they mean by strain?!

Ok, that’s a LOT to cover, so I’ll try to keep it all simple. First, let’s define what zoonosis is – this is a hot topic for One Health.

Zoonosis

Zoonosis, or zoonotic disease, is any disease caused by a pathogen (viral, bacterial, fungal, or a lesser known prion infection) that can be transmitted between animals and humans. Once transmission occurs between even just ONE type of animal and humans, it is considered zoonotic. Jumping species is dangerous because 1. it is no longer a pathogen contained within a single species (or, at least within the non-human animal world) and 2. any new species it begins to infect is a species without an immunity to that pathogen. We witnessed this with SARS-COV-2 during the pandemic – humans had zero defenses, causing severe immune responses in certain individuals.

Zoonotic diseases are becoming more and more prevalent as the climate changes and as humans encroach further into wild spaces. Other anthropogenic activities are also contributing to the uptick in zoonosis as well (i.e. farming practices), but let’s leave it at climate change and co-mingling with wildlife for now. Some examples of zoonotic disease are rabies, lyme, dengue, ebola, and also….certain influenza viruses.

Influenza

The flu is caused by a virus called orthomyxovirus (genus). This produces four species of influenza: A, B, C, and D.

Influenza A (IAV) is a zoonotic virus and has the most strains (subtypes) associated with it. These viruses are classified (named) after their specific proteins (H and N) giving us things like H1N1. This is also the species that causes our pandemics and epidemics (globally and regionally) and what most people think of when they think of the flu.

Influenza B (IBV) is not zoonotic. It affects only humans and has only two subtypes, which, in this case, are actually “lineages” (Yamagata and Victoria). Symptoms are generally similar to Influenza A with a similar potential to cause epidemics, though no pandemic has ever been caused by IBV, as far as we are aware. The yamagata lineage has also not been identified since 2020, which is curious. Perhaps it is lurking in the shadows, awaiting a comeback tour.

Influenza C (ICV) is mostly considered a human-only virus, but has been documented in other species, with very rare occurrences of human-swine transmission. Technically this would make it zoonotic, but transmission is so low, and symptoms are so mild that it rarely gets characterized or counted. [Caveat: respiratory symptoms in children under two years of age can be more severe, likely from new and developing immune systems.]

Influenza D (IDV) is a non-human animal virus found predominantly in cattle (bovine and swine, bovine being a primary reservoir) with a few spillover events into other animals such as sheep and horses. Serology samples, however, did show a high seroprevalence of IDV in farm workers with direct contact to cattle (bovine) and a relatively low prevalence in the general population, showing a transfer between humans and animals, especially those with close and continuing contact. Another study also showed a moderate increase over time of antibodies to IDV in a population in Italy (correlated to higher IDV prevalence in swine). It is worth noting, however, that there is high conservation of a specific protein between IDV and ICV and it was unclear if these individuals presented with illness. Seroprevalence is the detection of an antibody against a certain pathogen.

Flu Vaccine

So if people develop antibodies, why do we need vaccines? Some pathogens only elicit a short-term immunity, so it requires inoculation with the antigen (the disease-causing protein of a pathogen) periodically to give our goldfish immune memories a reminder of how to handle the virus when we encounter it.

The flu vaccines each year are bi- or trivalent against Influenza A (say, H1N1 and/or H3N2, broadly speaking) and B (Victoria) strains. The World Health Organization monitors flu cases through sampling and surveillance tools and determines which strains are most likely to cause widespread illness. They work with 152 national influenza centers located around the world. Additionally, in the US, panels of experts (committees) convene to chose a course of action. Those committees in the U.S. fall under the FDA, known as the Vaccines and Related Biological Products Advisory Committee (VRBPAC) and the CDC, known as the Advisory Committee on Immunization Practices (ACIP) (it should be noted that RFK Jr. recently removed all of the ACIP members and replaced them, citing possible conflicts of interest among the original members, a claim that has not been substantiated).

There are a few different types of technologies for developing the vaccine, which I won’t go into based on the length of this post already, but essentially, whether a live, but weakened form of the virus, a dead with still in-tact antigen form of the virus, or a vector (such as a bacterium that can’t harm us) containing the (DNA or RNA) code for the antigen, the point is to give the body a sampling of the protein that excites the immune system so that it slowly builds up antibodies.

Common Misconceptions

Just because you got the flu shot, doesn’t mean you can’t get the flu. Other strains, outside of the strain causing wide-spread illness, don’t just disappear or yield the floor to “the chosen strain(s)”. The world is constantly filled with pathogens, all of which don’t acknowledge human public health measures. We do the best we can to protect our populations and ourselves with expert guidance, the latest scientific knowledge, and vaccine technology.

Additionally, the flu virus is also not the only upper respiratory virus out there and getting the flu shot won’t help protect against those other viruses, necessarily. This isn’t like World War Z where the sick could walk through a hoard of zombies, unseen. You still need to practice other hygiene measures. That said, there is always potential for something called cross-immunity, or cross-reactivity in immunological terms, whereby you confer immunity to a different, unintended pathogen from the vaccine. This can happen if the antigen receptors (epitopes) are similar or the same as the target virus from the vaccine. But…..I wouldn’t count on this for my own health. Just wash your damn hands and cough/sneeze into your shirt!

Final Thoughts

Why is any of this important? Well, I mentioned earlier that zoonotic diseases are on the rise and that IAV is the most problematic of the flu viruses, the only one that is undoubtedly of the zoonotic variety. It includes bird flu and swine flu and these are spaces that overlap extensively with humans. Our tragic farming practices create unwell chickens and pigs, who are fed unhealthy diets, and left to suffer for slaughter in abattoirs that often breed other diseases. All of this has not only given us animals that are more susceptible to infection, but environments that are conducive to viral mutations and even the birth of antimicrobial bacterial strains. Remember those other anthropogenic causes I mentioned? They are linked so deeply with our agricultural and natural environments. If those spaces are unwell, then so is the life within them. Healthy environments tend to lead to healthy life. Apart from changing our practices, vaccines are some of the best defenses we have.

These last two posts have been pictureless walls of text, and I realize that is less fun to read. But I hope they have been helpful, regardless! Perhaps I need to break up the monotony with some actual STORIES next! Either way, thank you for reading and indulging a moment to nerd out just science.

Happy Friday!