mutations in influenza viruses occur frequently because the virus' replication machinery does not have a proofreading mechanism. When such changes cause mutations in the antigenic sites of the HA or NA genes, which reduce or inhibit the binding of neutralizing antibodies (the basis for resolving infection in an individual), the virus may evade the immune system.
This process explains the occurrence of seasonal influenza epidemics that may differ in severity and age groups affected. It also accounts for vaccine mismatch when one of the strains selected for the vaccine does not optimally match the circulating strains. As a result, annual changes in the composition of influenza vaccines are necessary.
Antigenic drift in the NA can lead to resistance against neuraminidase inhibitors. Antigenic drift may also allow a virus to cross the species barrier to a new host.
Antigenic shift :
Of greater public health concern is the process of antigenic shift – also called reassortment – through which at least two different viruses combine, resulting in exchange of the HA (for example H3 replaced by H5) and consequently the formation of a mosaic virus. This may happen when two different influenza viruses infect a cell and the genome segments are exchanged during replication.
Viruses arising as a result of antigenic shift may cause pandemics, since they appear suddenly in populations that may have no immunity and against which no existing vaccine may confer protection.
In addition to antigenic shift, a pandemic virus may arise by the process of adaptive mutation in, for example, a virus that has crossed the species barrier, such as avian influenza A(H5N1) infections of human beings. Adaptive mutation may increase the capacity of the virus to bind to human cells during subsequent rounds of infection, thus increasing its transmissibility among human beings.
For your info, covid-19 is also known as Sras-2, is said to come from Pangolin and is reported to cross species, some people claim cats and even a tiger have been infected.
So, for ALL we know right now, the thing is probably a mutation itself of something that was already there and will MOST likely followed by something else.
SARS 2002, H1N1 2009, MERS 2012, Covid-19 2020. There will be more. The herd-immunity strategy is born in think-tanks in the 2010s but was never actually used anymore and it didnt account for any of the specificities of this virus, very long incubation times, massive spreading which resulted in the overhelming of every country's healthcare system.
Letting 60% of your population get infected on purpose of a deadly virus that may mutate in the future is retarded af, full stop. Specially when a vaccin might be around the corner.