W ith the worldwide spread of COVID-19, which began in November-December 2019 and unfolded into a pandemic in the following months with the number of confirmed infections scaling the two-million mark and consequent deaths hitting 1,50,000, it was widely hoped that higher temperatures and increased humidity during the summer months would bring down the strength of the causative virus, SARS-CoV-2.
A recent study, submitted to the White House by the Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats of the U.S. National Academy of Science, Engineering and Medicine, has cautioned against pinning hopes on such a possibility. The report is based on a scrutiny of available evidence from recent investigations on the subject by different research groups. These works offered varying degrees of evidence, but, analysed together, the evidence is not sufficient to make a conclusion unequivocally one way or the other.
Countries of South and South-East Asia are already well into summer, and significant parts of India in particular are expected to experience heat waves. The India Meteorological Department (IMD) has forecast “normal to slightly above normal heat wave conditions… in the core heat wave zone [CHWZ] during the season”. So, in the light of the U.S. Academy study, India is unlikely to see any respite from the spread of infection on account of the hot weather. That has to come only from other health-related measures that are observed both at a societal and individual level.
The Academy’s nine-page report of April 7, titled “Rapid Expert Consultation on SARS-CoV-2 Survival in Relation to Temperature and Humidity and Potential for Seasonality for Pandemic COVID-19”, said in its summary: “[A]lthough experimental studies show a relationship between higher temperatures and humidity levels, and reduced survival of SARS-CoV-2 in the laboratory, there are many other factors besides environmental temperature, humidity, and survival of the virus outside of the host, that influence and determine transmission rates among humans in the ‘real world’.”
The report observed that while there was some evidence to support a potential decline in the number of cases in warmer and more humid seasons, none was without major limitations. “Given that countries currently in “summer” climates, such as Australia and Iran, are experiencing rapid virus spread, a decrease in cases with increases in humidity and temperature elsewhere should not be assumed. Given the lack of immunity to SARS-CoV-2 across the world, if there is an effect of temperature and humidity on transmission, it may not be as apparent as with other respiratory viruses for which there is at least some preexisting partial immunity,” it said.
It further noted that pandemic influenza strains had not exhibited the typical seasonal pattern of endemic/epidemic strains, thus emphasising that disease in a pandemic assumed a different pattern of spread and pathogenic virulence. “There have been 10 influenza pandemics in the past 250-plus years–two started in the northern hemisphere winter, three in the spring, two in the summer and three in the fall. All had a peak second wave approximately six months after emergence of the virus in the human population, regardless of when the initial introduction occurred,” it said.
Corresponding to the two questions that the academy assessment set out to answer— (1) survival of the SARS-CoV-2 virus in relation to temperature and humidity; and (2) potential for seasonal reduction and resurgence in the number of infections—the report divided the research work analysed into two categories: (a) laboratory experiments that involved deliberate dispersal of laboratory-grown virus under controlled environmental conditions and subsequent sampling; and (b) what it called “natural history studies” which looked at disease transmission in different locations and times of the year and sought correlations with environmental conditions such as temperature and humidity.
Although environmental conditions could be controlled in experimental studies, they almost always failed to adequately mimic those of the natural setting, the report said. On the other hand, in natural history studies the conditions (naturally) reflected the real world, but there was practically little control of environmental conditions and there were too many “confounding factors”.
Giving an example of the relevance of laboratory conditions to real-world conditions, the report said that many of the experimental survival studies used virus grown in tissue culture (TC) media. “Virus from naturally infected humans,” it pointed out, “when directly disseminated to the nearby environment has different survival properties than virus grown in TC media, even when the latter is purified and spiked into a relevant human body fluid such as saliva.” But it also noted that environmental dispersal of clinically relevant human fluids, such as saliva, respiratory (including nasal) mucus and lower respiratory tract airway secretions, urine, blood and stool, will be more predictive of the real-world situation than environmental dissemination of TC-grown virus in TC media.
According to the report, the laboratory results assessed so far indicated reduced survival of SARS-CoV-2 at elevated temperatures. This temperature sensitivity also depended on the type of surface on which the virus was placed. “However,” the report said, “the number of well-controlled [laboratory] studies available at this time on the topic remains small.”
As for “natural history studies”, the report said that published research so far have had conflicting results regarding potential seasonal effects. They are also hampered by poor data quality, confounding factors, and insufficient time since the beginning of the pandemic (which raged mostly in temperate regions during the winter months) from which to draw conclusions.
“There is some evidence to suggest that SARS-CoV-2 may transmit less efficiently in environments with higher ambient temperature and humidity; however, given the lack of host immunity globally, this reduction in transmission efficiency may not lead to a significant reduction in disease spread without the concomitant adoption of major public health interventions,” it added. Significantly, the report also pointed out that the other coronaviruses causing potentially serious human illness, including SARS-CoV and MERS-CoV, had not demonstrated any evidence of seasonality following their emergence.
According to the report, problems with data quality in the “natural history studies” included the estimates of reproductive rate, assumptions about infectivity period, and short observational time windows. Importantly, the report pointed out that there were other issues relating to the geography of locations with higher temperature and humidity that were studied: access to and quality of public health and health-care systems, per capita income, human behavioural patterns, and the availability of diagnostics. “As a reflection of these confounding factors,” it said, “those studies that show a significant correlation between temperature, humidity and disease transmission, also show that the two factors explain only a small fraction of the overall variation in transmission rates.”
In contrast, a latest study by W. Luo and others has argued against any seasonal differences from its findings which show sustained transmission despite changes in weather in various parts of China with climates that ranged from cold and dry to tropical.
However, this work, too, suffers from the same limitations as others with regard to data collection and case reporting. The authors of the study conclude that changes in weather alone will not necessarily lead to declines in cases without extensive public health interventions.
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