By Kevin Roche
Research results are flowing in on coronavirus and a lot of it is critical information on transmissibility. How the virus is transmitted, its hardiness in air or on surfaces, what “dose” is enough to cause infection; all of these questions help us understand how infectious the virus is, and one of the implications of a high level of infectiousness is how much of the population has already been exposed and infected, even without a positive test. (CDC Article) An article published at the Centers for Disease Control site describes a new analysis of transmissibility. Transmissibility stats and formulas are by their nature arcane and early estimates suffer from the usual completeness and accuracy of data issues. Initial work based on China had suggested a transmissibility number of around 2.5 (the number of additional people each infected person infects) and a doubling of infected persons every 6 to 7 days. The new research from China indicates that in fact transmissibility was much higher, around 5.7. New and more complete data allowed creation of a more accurate model of the spread. They also found that the likely incubation period from exposure to symptom onset was about 4 days, but this may be skewed by more seriously ill persons presenting earlier after infection. The higher transmissibility rate implies good and bad news. The good news is that it is likely that a much higher proportion of the population has been exposed and infected than would be indicated by positive tests alone. The bad news is that it suggests that a higher level of vaccine and natural immunity is necessary to prevent transmission, something on the order of 80% of the population. So depending on your mitigation of spread strategy, you get there faster but you need more people infected or vaccinated. And they note again the importance of testing, quarantine and contact tracing when transmissibility is so high.
Another paper published at Medrxiv also deals with transmission rates and use of lockdowns. (Medrxiv Paper) This paper divides the population into four groups–susceptible, exposed, infected and recovered and explores transmission dynamics among these groups and the effect of strategies to mitigate spread. They considered two primary transmission methods: person-to-person or contact transmission; and airborne or non-contact transmission. We don’t know a lot about either method, but we know more about person-to-person transmission. But it appears that the transmissibility number obtained from only including contact transmissions understates what we are observing so there must be some level on non-contact transmission occurring. Next they do an analysis about how much the transmission number must be reduced to keep cases below a country’s level of health resources. The higher your level of resources, the less you need to suppress transmission, if you are only concerned about keeping cases under the resource capacity. If non-contact transmission plays a role in a substantial number of infections, you should be focussed not only on social distancing, but on better hygiene in regard to non-contact transmission. And the authors again point out what should be obvious, the more you suppress infection in the first “wave”, the stronger the resurgence will be after suppression methods are lifted or lessened. Of course, one big problem is that we don’t know how many people are actually susceptible to infection after exposure or how many people are infected. We do know that a small proportion of the population is actually likely to suffer serious illness. So scientists should be exploring whether it wouldn’t be possible to attempt to suppress spread among those populations while allowing the rest of the population to resume normal activities, with hygiene, testing, contact tracing and quarantine measures in place.
Other research notes include that it appears, as I have suggested must have been the case, that the virus was circulating, perhaps widely, in the United States for several weeks before the initial cases were actually identified. Scientists detected this by analysis of the virus’ mutations. It appears that in NYC, many of the cases derived from travelers coming from Europe. Might be worth recalling that following the federal travel ban on people from China, both the Governor of NY and Mayor of NYC said they weren’t going to be racist or xenophobic and kept their city and state open to travel from Europe and other places for some extended time. The population paid a price for that virtue-signaling.