Author: Sara Habibipour
It’s flu season.
If you aren’t sick, everyone around you probably is (especially if you live in a college residence hall like me). One of the main culprits we have to watch out for this time of year is the influenza virus, better known as the flu.
What is the flu?
Like rhinoviruses (which cause the common cold), influenza is an RNA virus that manages to wreak havoc with very little genetic information–just 13 genes! The virus spreads through respiratory droplets of those already infected, such as through runny noses, sneezes, coughs, etc. Once a flu virus makes its way into your nasal cavity, it can make its way in through the cell lining of your airway. You begin to feel the common flu symptoms (fever, fatigue, coughing, chills, etc.) once the virus begins to attack the mucosal lining of the airway.
Are all flu viruses the same?
There are four types of influenza viruses: influenza A, B, C, and D. Influenza A and B are known to cause seasonal epidemics of the disease, but Influenza A is the only virus known to cause pandemics of the flu. Influenza C doesn’t tend to infect people that often and Influenza D mostly affects cattle.
Influenza A is divided into two subgroups based on the proteins that cover its surface: hemagglutinin (H) and neuraminidase (N). There are 18 different types of H and 11 different types of N. The most popular subtypes of the Influenza A virus are H1N1 and H3N2.
There are over 130 different H and N combinations that have been identified in nature, originating back to birds. This is due to a process called “reassortment.” Reassortment is when influenza viruses swap gene segments. One way this can occur is when two different types of flu viruses infect a person at the same time and exchange genetic information. For example, in the 1990s, pigs from both Europe and North America were imported to Mexico, each carrying its own version of H1N1. Due to reassortment, those two kinds of influenza mixed their genes, eventually creating the H3N2 subtype (which, by then had made its way to birds). It’s estimated that this tri-hybrid subtype circulated in pigs in Mexico for years, and it eventually made its jump to humans in 2008. It’s frightening for public health workers and doctors anytime there’s a new flu subtype because it’s impossible to predict the damage it might do.
Influenza B viruses are not divided into subtypes. Instead, they are classified into two lineages: B/Yamagata and B/Victoria. Additionally, Influenza B viruses mutate slower than Influenza A viruses.
Why do we need to get the flu vaccine every year?
The H and N proteins mentioned above are more specifically known as antigens; they are recognized by the immune system and can trigger immune response, such as antibody production.
However, when two flu viruses are antigenically different (ex: H1N1 vs. H3N2), the antibodies produced for one infection or vaccination will not recognize and neutralize the other. In other words, if you’re immune to one virus, you will not be immune to the other if they are antigenically different.
It is possible that two viruses can be antigenically similar even though they have different genetic composition. In this case, as long as you have established immunity to one virus, you will be immune to both viruses, as long as they have the same antigens on their surface.
Because of how often reassortment happens, and how often the antigens on the surface change, we have to get a flu shot every flu season to keep our defenses up to date.
So, how do scientists know which vaccines to develop if they don’t know which subtype of influenza will dominate flu season? Well, scientists are constantly collecting viruses from patients around the world. They read the sequences of their genes and observe what mutations are arising; based on this information, they’re able to develop a vaccine that will protect against newer variants.
What populations are most affected by flu season, and why?
The CDC recently conducted a study where they looked at severe flu-outcomes across 10 flu seasons in order to see which racial and ethnic groups were being most impacted by the virus. Overall, Black people had the highest flu-related hospitalization rate, followed by American Indian or Alaska Native (AI/AN) and Hispanic people. These trends were similar for ICU admission rates.
The study also found that disparities were greatest in the younger age groups, with rates of severe outcomes being up to 4 times higher among minority children ages 0-4 compared with non-Hispanic white children.
These facts are interesting because they seem to correlate with vaccination rates among racial and ethnic minorities. Since 2010, flu vaccine coverage has been consistently lower among Black, Hispanic, and AI/AN adults. During the 2021-2022 flu season, flu vaccination coverage was 54% among White adults, but only 42% among Black adults, 38% among Hispanic adults, and 41% among AI/AN adults.
There are many reasons for these inequities, including lack of access to healthcare and insurance, not being aware of the opportunities to be vaccinated or its significance, and misinformation and distrust in the medical system. As of now, the CDC is working to address these problems through the Partnering for Vaccine Equity program which provides funding to national, state, and local governments to improve vaccine programs in the communities of racial and ethnic minorities.
We never know when the next flu pandemic will be, only that it’s coming. Therefore, it’s vital that we all do our part to stay healthy and get vaccinated every year so that we can protect ourselves, as well as the populations around us who are more susceptible to the virus.