Viruses Don't Necessarily Become Less Deadly Over Time
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Understanding Viral Virulence
The term 'virulence' describes a virus's capacity to cause disease. A prevalent myth suggests that viruses inherently become less virulent as time goes on. For instance, there is a belief that HIV, the virus responsible for AIDS, is becoming less lethal, partly due to the evolutionary pressures from antiretroviral treatments.
The Concept of Declining Virulence
The idea of the 'law of declining virulence' originates from Theobald Smith, an American scientist who presented his theories in the late 19th century. Essentially, Smith's concept posits that pathogens, including viruses, tend to evolve in a way that reduces their harm to hosts, as a severely ill or deceased host is less effective at spreading the virus. In evolutionary terms, a pathogen that fails to effectively transmit decreases its fitness.
The Trade-off Hypothesis
Is it true that higher virulence always diminishes a virus's fitness? Not necessarily. A virus may still spread effectively even if its host is gravely ill. For example, in the case of SARS-CoV-2, the interval between infection and death is relatively long, allowing the virus ample time to replicate and infect others before the host succumbs. This line of reasoning, known as the 'trade-off model,' was articulated by Australian scientists Anderson and May in the 1970s, countering the earlier 'law of declining virulence.'
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The implications of the trade-off model are significant: it is incorrect to assume that greater virulence automatically lowers a pathogen's fitness. Each pathogen's fitness is contingent upon its unique interaction with the host. Thus, we cannot make sweeping generalizations about changes in viral virulence over time; we must evaluate each pathogen-host relationship on a case-by-case basis.
COVID-19 and Its Virulence
What can we say about the ongoing pandemic? As stated by Jemma Geoghegan in the Smithsonian Magazine, ‘the novel coronavirus is already quite fit’; its current virulence appears advantageous to SARS-CoV-2. Notably, we are witnessing variants like Delta that are believed to be more virulent and may elevate hospitalization risks for those infected.
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Are there historical precedents from past coronavirus pandemics that might inform our understanding of SARS-CoV-2's future? One such example is the coronavirus OC43, which is thought to have caused a pandemic in 1890, resulting in over a million deaths. Interestingly, OC43 was once linked to severe neurological effects but has since evolved to be one of the viruses responsible for common cold symptoms.
While this offers a somewhat reassuring perspective, Edward Holmes from the University of Sydney cautions against making sweeping predictions: ‘Attempting to foresee virulence evolution is a fool's errand.’ Pandemics conclude not necessarily because the pathogen becomes less virulent but due to increased immunity and containment measures. This insight provides a glimmer of hope for better days ahead, regardless of whether SARS-CoV-2's virulence diminishes.