Cooler and drier climates could have been more conducive to the transmission of Covid-19 virus, but available evidence does not suggest that weather conditions played a prominent role in the propagation of the virus, an expert group set up by the World Meteorological Organisation (WMO) has concluded.
In its final report released on Tuesday, the expert group said that “high-quality” research studies indicated a “generally negative association” between temperature and Covid19 transmission, meaning lower temperatures probably helped transmission of the virus. There was a similar correlation between humidity and Covid19 transmission, which meant that drier conditions were likely to have aided transmission.
However, the evidence was not very strong and the associations between weather conditions and transmission of the virus were probably complex and non-linear, the group said.
The expert group, or the international task team as it was called, was constituted by the WMO in September 2020 to assess whether meteorological factors like heat and humidity had any role to play in the transmission of the Covid19 virus. The team was also asked to study whether air pollution was leading to more severe diseases amongst infected people.
At that time, there were suggestions that like some other viral respiratory diseases, Covid19 might also acquire seasonality, particularly in cold and temperate regions, if it persisted for a few years. There was some laboratory evidence that indicated that virus had a greater chance of survival under cold and dry conditions. Prior experience with diseases like influenza pointed to the possibility of winter-time surges of the disease, especially in countries having cooler climates.
The relatively low prevalence of the disease in Africa and other tropical regions, at least in terms of known infections, also buttressed this theory.
Similarly, evidence from other respiratory diseases suggested that exposure to polluted air could increase the severity of the disease from Covid19 infection and possibly also raise mortality risks.
The expert group had submitted its first set of findings in March 2021 when the pandemic was still raging in most parts of the world. But even at that time it had said that meteorological factors had, at best, only a supporting role to play in the transmission of the virus. It had pointed to some research findings that had indicated that air pollution did in fact increase severity and mortality, but stressed that there was “no direct, peer-reviewed evidence” of these.
On Tuesday, the expert group submitted its final report. It has reiterated its earlier finding that the transmission of the virus was influenced more by government interventions, behavioural choices of people, vaccination efforts, and virus’ own evolution into different variants than by meteorological factors like heat or humidity.
“There has been no evidence that certain weather conditions (e.g., warm and humid conditions) absolutely precludes transmission (of the virus), as had been suggested by some commentators early in the pandemic,” it said.
“Overall, the literature provided evidence for possible associations between temperature and humidity and COVID-19 incidence, and poor air quality and severe COVID-19 outcomes. The findings suggest that associations between weather and COVID-19 incidence and severe outcomes are complex and likely non-linear. This complexity likely means that impacts of weather are dependent on the contexts of the studies in terms of geography, season, stage of pandemic etc,” it said.
But it warned that even if meteorological conditions and air pollution helped transmission or increased the severity of the disease by only a small amount, the fact that the number of infections and deaths are so huge, that their overall impact on the pandemic could be quite large.