Influenza is a very contagious virus that is susceptible to genetic variation. This makes flu season around the world dangerous since different strains appear and can easily infect the population. There are 3 types of influenza that affect humans, A, B and C. Influenza A is the type that is generally discussed when talking about epidemics and the most serious cases of the disease commonly come from this virus strain. Type B is essentially a milder version of type A and Type C is the most mild. The differences between these 3 types is the protein coat associated with the virus. Influenza is an enveloped single-stranded RNA virus with specific glycoprotein spikes on the envelope. The two types of spikes are either a hemagglutinin antigen (HA) or a neuraminidase antigen (NA). These antigens are what give each virus it’s pathogenesis and allows for it’s detection. Each variation is given a name (ie H1, H2, H3, N1, N2 etc) but of the 18 HA and 11 NA types we have found, only the aforementioned exampled 5 antigens can be passed between humans. Generally, the FDA’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) tries to match the flu shot with the strains they believe will be circulating most in the country. The vaccine only targets the HA spikes on the envelope of the virus. For the 2019-2020 flu season, the CDC believed that there were 4 main strains of the virus that would be most prevalent within the population.
In this season’s influenza vaccine, there is the A/Brisbane/02/2018 (H1N1)pdm09 component, the A/Kansas/14/2017 (H3N2) component, the B/Victoria V1A.1 and B/Phuket/3073/2013 (from the B/Yamagata lineage) virus components. This resulted in a quadrivalent, cell-grown vaccine for the season. The A components are the ones that have changed from last season and the B components are those that have stayed the same in the vaccine. As stated earlier, influenza is highly susceptible to variation which is why there are changes each year with the vaccine. The influenza virus can antigenically shift or drift. Antigenic drift refers to small genetic changes in the virus’s RNA code which leads to the differences seen in the HA and NA on the envelope. Antigenic shift refers to the way in which some parts of the RNA code of animal influenza can be taken into the human body and combined with our existing influenza strains, creating a new strain of the virus. This can only happen with influenza A and can be devastating and cause pandemics. This type of change is unpredictable and thus vaccines generally do not contain the strain leading to a lack of societal immunity. The CDC cautions that the the vaccine for the flu is only usually about 40% effective, but partial immunity is better than none and can result in a decreased severity of the flu if contracted.
The CDC gathers weekly influenza surveillance data which keeps the public informed of the results of the disease in society. From October 1, 2019 to February 1, 2020 there have been an average of 26.5 million illnesses, 12.5 million medical visits, 290 thousand hospitalizations and 21 thousand deaths. The difficulty this season is that there is a mismatch in the strains that were chosen for the vaccine and those that are circulating. As mentioned, Influenza A generally causes more severe cases of the flu and is the prevalent virus seen in society. Yet this year there was evidence an influenza B strain was called the B/Victoria V1A.3 was spreading more than the influenza A strains early on in the flu season. The fact that a strain of influenza B (not in the vaccine) is more common this season poses many risks for our population, specifically the younger population. It appears that most of the strains of Influenza that young children are affected by is the influenza B/victoria one because they have no previous experience with the strain. There is data supporting that when people have early exposure to a specific virus, they are able to better defend against it, something called immunologic imprinting. But the last time any B/Victoria strain of the flu was highly prevalent in our communities was the 1992-93 flu season, thus all children do not have any imprint of the virus in their immune systems are more susceptible to acquiring and having a severe reaction to the virus. Although the wrong strain of the B/victoria virus was picked for the vaccine this season many times even the wrong strain can provide some help and immunity. Thus even with this mismatch, people should still get the vaccine.
Microbiology Blog 