As our fearless leader said in the early stages of this disater, "“It’s going to disappear. One day — it’s like a miracle — it will disappear.” Well, that among all the other stupidity he has uttered.
But actually, I want to consider is the widespread belief that COVID-19 will exhibit a seasonal pattern, like influenza and certain other viruses. Some of that comes from the initial reports equating COVID with the 'flu' and colds, and the fact that there are human coronavirus types which cause a upper respiratory illness like the common cold. It's important to realize that there's no genetic clock in the influenza virus and no temperature sensitive gene that causes seasonality. Respiratory illness like colds and influenza are active year-round. Indeed, each year's US influenza vaccine is selected from strains circulating 6 months before in Aisa and the Southern Hemisphere.
Cases of colds and influenza increase and peak in the US in winter for social reasons. Influenza and colds are transmitted by droplets but mostly by contact. The infectivity of influenza is less than coronavirus and the survival of virions on surfaces appears to be shorter (maybe). So, in the winter, people are inside more, inside schools or work, and are on average physically closer together. For a more highly infectious agent with (apparently, but not proven) longer persistence on surfaces, like COVID, close quarters are not as necessary for transmission. Certainly relative isolation appears to slow transmission but contries which have been touted as successes in controlling the outbreak (Singapore, South Kore, and China outside of Hubei, for example) are now experiencing a slower but inexorable accumulation of cases.
In the case of SARS and MERS, the two prior coronavirus outbreaks, There's too little data to infer seasonality. SARS was first detected in China in Nov 2002 and spread in China and elsewhere from March-July 2003 (timeline). Measures such as early travel restrictions and aggressive contact tracing seemed to be effective. MERS was first detected in Jordan in April 2012, spread within the Arabian peninsula during 2013 and travel-related cases were reported in the US and Europe in May 2014 (Wikipedia). 80% of cases were in the KSA. Both the illness exhibited similar features to COVID including high infectivity, spread within hospitals, and led to servere illness and deaths in health care workers, but no clear seasonality was seen.
Rapid spread of flu and colds is limited as many people have some immunity to influenza from prior infections. Usually, seasonal flu varieties have no or slight differences from year-to-year, so-called antigenic drift. Only rarely will a totally new type like H5N1 or the 1918 pandemic virus arise ("antigenic shift"). In that case, the attack rate will be much higher and, as in the case of the 1918 epidemic, cases will continue into the summer may further escalate in a second wave the next fall. Then, as the proportion of recovered persons in the population increase, transmission of the virus from person-to-person is decreased as susceptible persons are less likely to contact active cases. Most models of herd immunity require >80% immunity in the population for effective prevention but that varies with the infectiousness of the pathogen. Indeed, if you consider the measles outbreaks in the US and other developed countries, even vaccination levels that drop below 90-94% can lead to outbreaks.
Herd immunity assumes that 1. the contagiousness of a patient is self-limited and recovered patients do not continue to spread disease after clinical recovery, 2. the recovered patients (or vaccinated individuals) have immunity to infection or re-infection. Both of these assumptions have been called into question now due to the observation of clinically recovered patients testing positive for COVID-19 RNA weeks later. At this point, given the widespread community transmission in the US, it is likely that the role of contact tracing is very limited. In fact, the most likely outcome is that most (>90%) of Americans will be exposed and infected and the outbreak will smolder on for years.
Keep safe: wear a mask, wash your hands, and minimize cotact with groups of people. Locally, I would estimate that >10% of people are potentially infectious at this point.
Posted by Gordon, No Hair Blog, April 12, 2020
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