In my previous article, I looked into some recent research on influenza transmission and raised the possibility that a similar pattern of transmission hypothesized for it may hold for SARS-CoV-2 (SCV2). As a quick reminder, influenza researchers have recently found that a much smaller dose of virus was needed to infect people if it was delivered in the aerosol form than if they were inoculated intranasally. More recently, other researchers conducted a human challenge study in which they split a group of volunteers into the donors who were to be infected directly and the secondary attack group that was to be infected by the donors. The intranasally infected donors managed to infect only one person, despite supposedly favourable indoor conditions with low humidity.
The researchers hypothesize that, in order to spread effectively, influenza may require either really poorly ventilated indoor spaces and (or) a minority of superspreaders with lower respiratory tract involvement. The evidence for SCV2, while less elaborate, seems to point in a similar direction. Chinese and German researchers have found essentially no evidence of it spreading outside, and the evidence of it spreading like wildfire in closed quarters from cruise ships to nursing homes to prisons is too overwhelming to require restating here.
Another intriguing recent finding may point at the LRT-involvement superspreader hypothesis for SCV2. The Japanese researchers who studied the lung scans of the infamous Diamond Princess cruise ship’s passengers found that even many of the supposedly asymptomatic cases had signs of significant abnormalities. There are two possible interpretations here. Either almost all asymptomatic people infected with SCV2 have LRT involvement, or many people had it on the Diamond Princess because the cruise ship clientele skews older and the viral doses involved may have been abnormally high because of the cruise ship conditions. If the latter hypothesis is true, it points at the same thing in play as the recent influenza research.
The infection fatality ratio
Another area where we are increasingly seeming a commonality between SCV2 and influenza is in their actual lethality in the general population. The indicator of interest here is what is formally referred to as the infection fatality rate (IFR), or the number of deceased from the virus divided by the total number that it has infected in a representative population.
The IFR should not be confused with the case fatality ratio (CFR) which has a completely different denominator, namely, the number of confirmed positive cases. Sadly, the CFR numbers in many countries have been badly misused by the supporters of extreme measures to curb Covid-19. Even though it was clear from the start that CFRs have been persistently high (especially in a bunch of particularly badly hit countries) because of the low testing targeting mostly hospitalized people with the infection.
High CFRs from various countries have created a strong impression that Covid-19 was highly fatal for a highly transmissible virus, causing a lot of unjustified panic. The WHO, that really should know better, claimed at some point that the virus could kill 3.4% of the infected. However, evidence from mounting serological studies (flawed though each of them may be individually) is increasingly pointing at a prevalence of the infection that is way higher than what confirmed cases suggest. It increasingly appears that while perfect storm hotspots like Lombardy and NYC may have IFRs at around 0.5, in other places, they will end up to be close to 0.1 estimated for influenza.
It is also worth noting that Covid-19 will probably never again be allowed to ravage nursing homes the way it has done in early 2020. And the reports of many asymptomatic or mildly symptomatic cases even in nursing homes suggest that the virus may have killed most susceptible people (whether it is genes, lack of recent exposure to milder human coronaviruses or something else that makes people susceptible). Even if another major outbreak strikes, its mortality will be much lower just because of this.
It should also be taken into account that influenza pandemics — even when not particularly lethal (unlike the Spanish flu) — have sometimes produced unusually high mortality in some places. For instance, the little-known 1951 flu pandemic hit the UK particularly hard for some reason. In Liverpool its fatality rates may have exceeded those seen in the Spanish flu.
One can of course retort that something like Spanish flu was probably quite lethal. However, the leading hypothesis of its high mortality is bacterial superinfection in the pre-antibiotic era coupled with the immediate consequences of WW1.
The spread and the climate
The final intriguing commonality may have to do with how influenza and SCV2 spread depending on the climate. SCV2’s pattern in this regard may look very confusing. It predictably hit hard many areas with relatively low temperature and low humidity in the Northern hemisphere from Seoul to Wuhan to Milan to Madrid to NYC. It seems to struggle to cause massive outbreaks in Florida or India but at the same time it also seems to ravage through coastal areas of Ecuador and cause devastation in places in central Brazil like Manaus. What is going on here?
Astonishingly, influenza may suggest the answer. It has been established for some time that for some reason, it behaves differently in temperate regions compared to the tropical ones. In the former, it tends to persist at high levels in the winter season, but in the tropics, it spreads much better in the very humid conditions of rain seasons.
It has already proven hard to explain why influenza spreads easier in cold and dry conditions in the temperate zones, and it is probably harder still to square the explanation with the enhanced tropical transmission at maximum humidity. However, possible explanations aside, this two-peak pattern appears to be well-established.
And there are tantalizing signs that it may hold for Covid-19, too. The most obvious question that comes to mind about Florida and India versus central Brazil and coastal Ecuador is when it rains the most there. And the answer does not disappoint. In coastal areas of Ecuador, the rain season is between January and April. In Manaus, Brazil, the month with peak precipitation is… April. Whereas in Florida and India, the rainy season starts in May and June, respectively.
A sad implication of this hypothesis if it is true, however, is that countries like India may have decided to crack down on Covid-19 at a completely wrong time, if lockdowns are ever appropriate. They will wreck their economies but the virus may hit hardest later.
P. S. Nothing in this article is to suggest that influenza and SARS-CoV-2 are completely the same. They are obviously from different virus families and they probably infect and replicate within cells differently. This means that one should not run to a pharmacy to buy an anti-flu medication after reading this article.