Possibly - also totally possible it's about the same amount of virulence, but our population has so much more immunity that we don't get nearly as sick. And this is true even in places where the virus has transmitted so much, unfortunately, in the past that there is a lot of population-level immunity even without high vaccination rates.
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Section Branding. Header Content Tagged as: News Health. Viruses evolve and weaken over time — what does that mean for the coronavirus? Primary Content. Thank you so much for having me this morning. Tags : Series: The Coronavirus Crisis. Bottom Content Related News. True or false? The influenza vaccine always remains the same from year to year. Article Menu [ ]. Vaccine Science [ ]. Biological Weapons, Bioterrorism, and Vaccines. Cancer Vaccines and Immunotherapy.
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Vaccines for Adults. Vaccines for Teenagers. Influenza is native to water birds, where like rhinovirus it has developed into many different strains. It has also spread to other host species, including people, pigs, horses, dogs, and others. Influenza has a feature that makes it especially easy for it to share its genes.
Its 10 or so genes are distributed across 8 separate segments. As new virus particles are built, one copy each of all 8 segments are packaged together inside. Particularly dangerous for the hosts is when an individual is infected by influenza viruses from two different species—say a human and a bird strain. Segments from the two strains can be packaged into new viruses in any combination.
The result is hybrid viruses that carry a combination of genes from two strains. Through reassortment, a bird strain can gain the ability to infect human cells. An event like this produced the virus that caused the influenza pandemic.
Together, ongoing mutation and reassortment give influenza a high level of variability. New strains develop each year, often with the ability to evade our immune system. Viruses that combine genes from animal and human strains are especially dangerous. They can be so different from other viruses that circulate in human populations that we have little immune protection against them. But this does not mean we are doomed.
There are ways to fight back, and everyone has a role to play. The most important thing you can do is keep viruses from spreading. When a virus can spread easily from person to person, the fastest-replicating viruses spread the farthest. Viruses acquire mutations that make them replicate even faster. This is usually very bad for the host because a fast-replicating virus can overwhelm their immune system. But this evolution toward speed is held in balance by an opposing force: When a virus kills its host too quickly, before it can spread, it reaches a dead end.
The virus may acquire mutations that slow its replication speed. This can give the immune system a chance to eliminate the virus. It also buys time for using other measures, like medications and other treatments.
Cauldwell, A. Viral determinants of influenza A virus host range. Journal of General Virology, 95 6 , Clutter, D. HIV-1 drug resistance and resistance testing. Infection, Genetics and Evolution, 46, Dou, D. Influenza A virus cell entry, replication, virion assembly and movement. Frontiers in immunology, 9, Other evolutionary biologists disagree.
Evolution always favors increased transmissibility, because viruses that spread more easily are evolutionarily fitter — that is, they leave more descendants. Some germs do just fine even if they make you very sick. The bacteria that cause cholera spread through diarrhea, so severe disease is good for them. Respiratory viruses, like influenza and the human coronaviruses, need hosts that move around enough to breathe on one another, so extremely high virulence might be detrimental in some cases.
Nor are there many documented instances of viruses whose virulence has abated over time. The rare, classic example is the myxoma virus, which was deliberately introduced to Australia in the s from South America to control invasive European rabbits.
Within a few decades, the virus evolved to reduce its virulence , albeit only down to 70 to 95 percent lethality from a whopping It has since ticked up again. But myxoma stands nearly alone, Parrish says. For instance, he notes, there is no evidence that recent human pathogens such as Ebola, Zika or chikungunya viruses have shown any signs of becoming less pathogenic in the relatively short time since jumping to humans. The faded nightmares of our past — pandemics that terrorized, then receded, such as SARS in and flu in and again in , and — went away not because the viruses evolved to cause milder disease, but for other reasons.
In the case of SARS, the virus made people sick enough that health workers were able to contain the disease before it got out of hand. Flu pandemics, meanwhile, have tended to recede for another reason, one that offers more hope in our present moment: Enough of the population eventually becomes immune to slow the virus down.
The H1N1 influenza virus that caused the pandemic continued as the main influenza virus until the s, and its descendants still circulate in the human population. What made the virus such a threat in is that it was novel and people had little immunity.
Once much of the population had been exposed to the virus and had developed immunity, the pandemic waned , although the virus persisted at a lower level of infections — as it does to this day. It appears less lethal now largely because older people, who are at greatest risk of dying from influenza, have usually encountered H1N1 influenza or something like it at some point in their lives and retain some degree of immunity, Read says.
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