Factors affecting COVID-19 infection and death

As many countries are emerging from COVID-19, they are trying to determine the true extent of infections and what lead to deaths, since most people survive.  For infections people turn to Ab test in the general population.  For deaths will need to comb through the records to find the ones missed during the chaos.  It is expected in most countries deaths will increase by a small fraction, but Ab test will indicate many times the official figures.

In numerous reported Ab test results (e.g., https://www.the-scientist.com/news-opinion/researchers-applaud-spanish-covid-19-serological-survey-67590), 1-10% population are positive, often 10x of the official confirmed case numbers.  Many countries have official fatality rates > 10%, so based on Ab results, the actual fatality rates drop to about 1%.  But there are caveats about Ab results.  Since Ab kits don’t get 100% specificity and 100% sensitivity, there will be false negative and false positives.  Considering most people weren’t infected, false positive might outnumber false negative.  On the other hand, it has been reported that not every infection produced Ab, adding to false negative as well.  Thus, Ab test gives only a rough estimate.

Still, if many more people were infected silently, what it means other than lowering the official fatality rate?  A scenario is that most infections were very mild, spread a bit, and then stopped.  And we may need to introduce a new category: abortive infection, which includes the asymptomatic (RNA positive but never ever any symptoms) and particularly those who never even get infected (perhaps RNA negative) despite inhaling the virus.  They still develop Ab which counts for virus elimination.  In other words, don't assume one positive (RNA or Ab) will always lead to more positive.  These people had no or few symptoms, and they didn’t infect others or such infections led to mild cases as well.  

As a coin has two side, this is also likely the major reason Wuhan didn’t find many COVID-19 cases by mid Jan 2020, despite hospitals had been on the lookout since late Dec 2019.  Then widely reported by Chinese news, few of the close contacts of patients exhibited any symptoms, leading to a wrong conclusion that COVID-19 was not infectious.  Now the explanations are: while most of them were probably not infected, some of the remainders were pre-symptomatic, and some were essentially asymptomatic throughout, and both, especially the pre-symptomatic, could transmit COVID-19.  The concept of pre- and asymptomatic, quite different from SARS, was established only in late Jan and early Feb.  Regardless, older patients are sicker.  And the sicker one is, the more infectious.  Vice versa.    

Fatality rates are not simple concepts and depend on who are infected (young or elderly) and how well the medical system is prepared.  In official statistics it also depends on how widespread testing is conducted.  But looking at the tallies of different countries one can already get a good understanding of COVID-19.  A conclusion can be drawn that warm weather limits infections and deaths.   

That summertime stops respiratory diseases is common sense.  Its effects on COVID-19, though, have been unclear, since COVID-19 is new, and countries in the southern hemisphere also experience infections.  But now there are more data, and by examining infections and deaths in “warmer” countries and “cold” countries in “warmer” times, it is clear that these countries do better in terms of their death figures and rates, cleaner parameters than the official infection figures.

The major Western European countries (UK, Italy, Spain, France, etc) started COVID-19 outbreaks in late Feb and early March, with official fatality rates >10%.  Southeast Asia started COVID-19 in late Jan-Feb, none reached the high numbers of infections and deaths.  For example, Thailand infection: 3.1K, death: 58.  Vietnam: 329, 0.  Cambodia: 125, 0.  Singapore: 37K, 24.  Singapore has two phases, the second phase started at the same time as Europe.  Even Australia, 7.3K, 102.  Australia has a sparse population, but other countries do not.  These countries are certainly better prepared for COVID-19, but it is also hard not to think hot weather blunted infections.   

Then Russia, outbreak a bit later than West Europe but was colder: 458K, 5.7K, and 221K recovered (June 6, 2020 data).  The high infections reflect wide testing coverage, and the ~1% death rate may be a factor of younger population.  Even 5.7/(5.7+221) gives <3%.  The fatality rate is higher than those in SE Asia but lower than West Europe.  The Russian figures do not support the warm weather hypothesis but do not contradict it either.    

India: 238K, 6.7K, 114K recovered.  The weather is hot.  India likely undertests its cases, is less prepared in terms of public health hardware, but may be helped by a young population.  Its fatality rate is much lower than the worst in Western Europe, supporting the warm weather hypothesis.  The same is true in Africa (https://www.huffpost.com/entry/developing-countres-coronavirus-united-states-global-south_n_5edaa785c5b6dc3de7b94546).  Both India and most African countries can claim better preparation, but is it enough to explain why they are doing much better than Western Europe and the US?  No one thinks warm weather is the 100% COVID-19 stopper, but maybe it does help 20%?

The last remaining piece is Brazil: 645K, 35K, 268K recovered.  This is an odd case, warmer weather (getting cooler though), and less intervention by the federal government.  

Overall, a trend is clear: if a country started COVID-19 outbreak in warm or hot weather, it would have a better outcome than a country in cold weather.  Of course there are other considerations, like how seriously a country was prepared.  South Korea and Japan are two good examples. 

Still, correlation does not mean causation.  In the US northern states like NY fared worse than southern states like FL, but what does it mean?  We need a mechanism: why warm weather thwarts COVID-19?  Similar to the case of cold/flu.  There are two major factors.  One is that in cold weather people congregate indoor, and in warm weather they stay outdoor more.  Common sense that too many people staying indoor increases the possibility of many people getting infected.  The other is that at outdoor, even if one gets the virus, the viral load is likely lower, because virus is more diluted.  Hence his or her immune system may be able to fight off COVID-19, leaving no actual infection or only a very mild disease, which is then harder to transmit to others.  Similarly, young people, who also tend to go out more, with a milder disease are less infectious than older patients as well.  So the novelty here is emphasizing the notion of viral load, reducible by warm weather and face masks (5/23/2020 blog).  Lower viral load, fewer infections, fewer deaths.

Expect to see papers on these soon, more detailed data and analyses, but same conclusions.  Most importantly, though, is what to do with this realization.  It means that countries in the north should reopen faster.  If social distancing is maintained, new infections will continue to drop during the summer.  Even with more infections, an inevitability, new deaths will decrease more.