The influenza vaccine is a trade off, like many things in our life. Scientists try to predict the strains that most people will be exposed to during the flu season and put those into the vaccine. For each strain added, we increase the cost of production and more importantly, each strain increases the load on your immune system. More strains might result in multiple shots needed, which decreases the number of people who get fully vaccinated. Or it could cause side effects, similar to what we have seen with the SARS-CoV-2 vaccine, which will decrease the number of people who get the yearly vaccine. Luckily, scientists are working on a universal influenza vaccine. This type of vaccine would vaccinate against a protein on the influenza virus that doesn't change. This would mean protection from more strains, as well as less frequent vaccination.

The quadrivalent inactivated influenza vaccine is targeted against currently circulating H1N1 and H3N2 influenza A viruses, and Yamagata and Victoria lineage influenza B viruses (4 circulating types of virus = 4 components of the vaccine). Universal vaccine approaches (as we’ve mentioned in responses to other questions in this session) are hoping to expand vaccine breadth not by including more individual HA targets, but by targeting conserved parts of the virus that are consistent across a wide range of virus subtypes.