Covid 19

The 2019/2020 flu vaccine included vaccines for four strains: A(H1N1)pdm09, A(H3N2), B/Victoria, and B/Yamagata.

Influenza viruses come in four types: A, B, C, and D. Type D is not known to infect or cause illness in humans (it primarily affects bovines). Type C does infect and cause illness in humans, but the symptoms are very mild, and type C does not cause epidemics. Types B and A both infect and cause illness in humans. Type B is only found in humans, generally causes a less severe reaction (illness) but can still be harmful, it is divided into two lineages (Victoria and Yamagata with each having groups and subgroups), and is known to be associated with epidemics. Type A can infect animals (which is why we have swine flu and bird flu) but generally only humans get sick while animals are incubators. Type A strains are identified by the proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N), of which there are 18 H and 11 N subtypes (but only 131 subtype combinations have been identified). Thus type A strains are identified by their protein numbers, ex., H1N1 and H3N2. Type A strains are responsible for most illnesses during the flu season and are associated with both epidemics and pandemics.

Influenza viruses, and viruses in general, are constantly mutating. Influenza and coronaviruses are both single-strand RNA viruses so everything said here should be considered applicable to them and not necessarily applicable to double-stand RNA viruses, DNA viruses, etc. Antigenic drift occurs when mistakes are made during copying of the viruses RNA. While these mistakes happen all the time not every mutation will result in antigenic drift. Further, because these changes are slow and cause small changes in the virus a person can have partial immunity even though the virus has drifted. Because a person may have partial immunity from a virus that has drifted this is why someone who did not get a flu vaccine might get the flu but only get a very mild case. Those people who never get a flu vaccine but only get sick every few years likely have carried over partial immunity. Influenza type B only undergoes antigenic drift. Antigenic shift is more rare and are rapid changes. These occur when the virus swaps part of its RNA and can affect a persons ability to fight the virus as there will be no carried over partial immunity. This is what happened in 2009 with H1N1/pdm09. Influenza type A undergoes both antigenic drift and, less frequently, antigenic shift. This is why flu vaccines have to be updated each year. For 2019/2020 both strains of type A had to be updated while both strains of type B remained the same as the prior year. Side note, the reason the A/H1N1 strain has "pdm09" appended to it is because it is the novel strain of H1N1 that emerged in 2009, it was very different from the H1N1 strain that was already circulating, and was responsible for that year's pandemic. Even though the 2008/2009 flu vaccine included H1N1 it was the existing strain and offered little protection against H1N1/pdm09, which is why 2009 was such a bad flu season.

Coronaviruses are quite different from the influenza virus. Coronaviruses make up a large family of viruses that generally only cause mild symptoms in humans and are one of the viruses responsible for the common cold (along with rhinovirus (responsible for ~50% of colds), respiratory syncytial virus (RSV), and parainfluenza (no relation to influenza)). There are over 30 types of coronavirus, with four subgroups, but only seven are known to affect humans. The four common types are 229E (alpha), NL63 (alpha), OC43 (beta), and HKU1 (beta). The three less common types, and the three known to cause more severe symptoms, are MERS-CoV, SARS-CoV, and SARS-CoV-2 (which causes Covid-19). Side note, Covid-19 is actually an acronym for Coronavirus Disease 2019. Coronaviruses do mutate and change over time and while we don't have quite the body of research on coronaviruses that we do on influenza viruses research points to coronaviruses changing slower than influenza viruses over time. While there are currently three known variants of the SARS-CoV-2 virus they are very similar to each other with only slight differences. These differences do not make them different enough to be considered different strains such as with H1N1 and H1N1/pdm-09. SARS-CoV-2 is undergoing antigenic drift, like influenza type B, and those changes occur slowly over a predictable time frame, often the same mutation can be seen in different hosts, and are generally small changes that make little difference between variants.

Because the mutational changes in SARS-CoV-2 are slight, effectively making the different variants indistinguishable, when a vaccine is produced we won't have to worry that some vaccinated people are protected while others are left vulnerable in a similar situation to the 2009 H1N1 Swine flu pandemic. The greater concern with be two fold. First will be ensuring that enough of the vaccine is produced and able to be distributed to produce herd immunity (enough people are vaccinated to act as a shield for those who are or can not vaccinate). Second will be those who refuse to vaccinate (wholly different from those who not able to vaccinate for medical reasons) and if their numbers will be low enough to allow for herd immunity. Frankly, with the number of protests that are occurring around the US, and the general anti-science and anti-big pharma mentality that pervades a particular segment of the US population, I fully expect that there will be pockets of unvaccinated people who will allow this virus to survive for a while.