COVID: ??seasonal variability

A recent modeling study suggested that the current Covid-19 pandemic will be minimally affected by changes in seasons, or climate drivers (see  https://science.sciencemag.org/content/early/2020/05/15/science.abc2535/tab-pdf , covid climate role science2020 in dropbox, or 10.1126/science.abc2535 )

 

Details:

-- increased specific humidity (the mass of water vapor in a unit mass of moist air) does decrease the transmissibility of influenza and respiratory syncytial virus (RSV), affecting their seasonality and the increased transmission/epidemics in the cool dry winters in northern latitudes

-- this is not true for all viruses: for example, enteroviruses often peak in summer months

-- emerging pathogens, on the other hand, have dynamics more driven by high population susceptibility and their intrinsic transmissibility (R₀ value)

-- we do not know the climate susceptibility of the SARS-CoV-2 virus, so the researchers looked at 2 common Coronaviruses that have the same betacoronavirus genus as SARS-CoV-2: the HCoV-H1KU1 and the HCoV-OC43

    -- they assessed the data on these coronaviruses from the U.S. Census regions to understand the potential climate dependence of betacoronavirus transmission

-- and, knowing the climate-dependence of influenza and RSV, they could use the population-weighted average climatology of specific humidity 2014-2020

-- they then assessed 3 scenarios, assuming the SARS-CoV-2 has the same sensitivity to climate as influenza, and assuming that SARS-CoV-2 has the same climate dependence and length of immunity as the 2 different betacoronaviruses above

-- they characterized the link between specific humidity in the transmission of SARS-CoV-2 using estimates from the other 2 beta coronaviruses as well as influenza

-- they did simulations of a pandemic, focusing on the results on 9 cities each with a very distinct mean and seasonal cycle of specific humidity

-- but, they did not address potential complexities such as demography, infection control measures, and other environmental factors

 

Results:

-- their modeling did show that in tropical locations there tends to be a more sustained but lower intensity pandemic than in the northern hemisphere, related to the decreased R₀ values in areas of high humidity there (though they do not take into account increased transmission rates related to population density)

-- and they found only minor delays in the peak of pandemics in the southern hemisphere vs the northern hemisphere despite the six-month shift in specific humidity

-- they also ran some scenarios for efforts of non-pharmacologic control (eg distancing, masks, etc), finding that this would lower the R₀, and:

    -- depending on how much the R₀ decreases, the time til the peak pandemic hit would increase (flattening the curve), but that also would increase the time to higher levels of population immunity, and therefore decrease the effect of humidity

    -- and, the higher the R₀, the quicker the immunity, and the more important a role that climate would play in determining pandemic peak size

 

Commentary:

-- there are several issues with this modeling:

    -- the climate dependency of the 2 recurrent common betacoronaviruses we have had are different, suggesting different dynamics driven by herd immunity (and that all betacoronaviruses don't act the same way)

    -- there may be complications related to cross-protection from other strains of coronavirus (they assume that each coronavirus has their own specific immunity)

    -- they did not include important factors, including population density, cultural factors that might increase viral spread (eg might be higher in countries with more social contact, such as more frequent hugging or kissing; and lower in areas where people naturally have more social distancing), or other potentially important variables (eg obesity, diabetes, etc: see http://gmodestmedblogs.blogspot.com/2020/05/covid-its-those-french-fries.html )

    -- they did not do separate analyses for other seasonal factors, such as temperature

    -- the comparison with flu is in fact pretty complex, since the immunology is pretty complex (at least to me)

        -- there are several studies finding pretty transient immunity from vaccines, though this does vary some by influenza type (eg see http://gmodestmedblogs.blogspot.com/2018/09/maybe-we-should-delay-giving-flu-vaccine.html )

            -- and, this would suggest lower levels of general immunity in subsequent years, making the annual flu epidemic more like a newly emerging infection with decreased seasonality per the above model/assumptions (which it doesn't seem to have)

            -- and, at least for the more recent flu pandemics (that i remember), there was no clear seasonality

        -- but, there are also studies in older people (who tend to have a less robust immunologic response) showing that repeated annual flu shots decrease their morbidity (see http://gmodestmedblogs.blogspot.com/2018/01/flu-vaccine-in-older-people-decreased.html )

        -- so, there seems to be a more complex immunology here, and this may vary perhaps depending on the quasispecies of influenza around. But the seasonality seems to be maintained

 

So, this might be a somewhat complex distillation of a more complex study, but the bottom lines from this reasonably sophisticated modeling study are:

-- there may be some effects of humidity in terms of the transmissibility of SARS-CoV-2

-- but, for a pandemic from an emerging pathogen with no population immunity, the principal driver of disease transmission is likely to be its inherent transmissabilty (R₀), with minimal further effect from humidity/seasonal variation

-- on the other hand, if there is a 2^nd or 3^rd or further peak of SARS-CoV-2, there will likely be higher levels of population immunity, lower levels of viral transmissibility (a smaller susceptible base), and seasonal factors may play more of a role

-- so, overall their model suggests that there we are likely to continue with Covid-19 through the summer..... 

geoff

 

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