Will summer halt Covid-19? Scientists say there are other factors
In a finding that has huge significance for India, where summer is just breaking, a prestigious scientific panel in the US said that high temperature is unlikely to significantly stop the spread of the coronavirus disease (Covid-19) without major public health interventions, such as personal protection and social distancing.
The report is from the US National Academies of Sciences (NAS), which, on April 4, said Covid-19 can be spread just by talking or breathing, resulting in many countries, including India, to revise guidelines on masks.
With countries currently experiencing summer, such as Australia and Iran, experiencing rapid virus spread, a decrease in cases with increase in humidity and temperature should not be assumed said the report, which examined virus survival in relation to temperature and humidity, and potential for seasonal reduction and resurgence of cases to provide scientific evidence for decision-making.
Thus, while noting that “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 the host, that influence and determine transmission,” the report said in its summary.
The report also calls for more research into the theme, including on various SARS-COV-2 strains.
The new report, which was submitted to the White House on Tuesday, said that on current evidence and given the lack of host immunity globally, any reduction in transmission because of higher temperature and humidity may not lead to a significant decrease in disease spread without major interventions, such as personal protection and social distancing.
“We cannot rely on summer to rescue us from coronavirus disease as temperature is just one of the many factors that affect transmission. Very hot climate may reduce cases by a small number in high population density areas as heat will destroy the virus faster. I’m hopeful it may do so to some extent in Dharavi, but since hotspots are typically in hospitals, malls and offices, which are often air-conditioned even in India, transmission is unlikely to end in summer without strong public health interventions,” said Dr Anurag Agrawal, director of the Institute of Genomics and Integrative Biology, New Delhi.
Other coronaviruses that cause potentially serious human illness, including both Sars-CoV and Mers-CoV that have caused outbreaks of severe acute respiratory syndrome and Middle-East respiratory syndrome in the past, have not demonstrated seasonality either. Indeed, a reading of the history of such illnesses shows that those that first emerged in winter, saw a second wave in summer - and vice versa.
Some of the compounding factors that affect virus transmission are geography, population density, per capita income, access to testing, quality health care systems and human behavioural patterns, including mobility and migration.
“There are many other factors that may influence virus spread and it is still too early to draw definitive conclusions without data on this novel virus in the real world. In the Indian context, the factors would include population density, difficulty of maintaining physical distancing in households where space is limited, inability and unfamiliarity with cough/sneeze etiquette, migrant labour returning home with the virus, and mingling of people even during the current lockdown,” said Dr V. Ramana Dhara, director, Indian Institute of Public Health, Hyderabad.
With little evidence on how this new virus will behave, virologists are falling back on the behaviour of coronaviruses, particularly SARS-CoV, with which SARS-COV-2 shares the most similarity.
Pandemic influenza strains have not exhibited the typical seasonal pattern of endemic and epidemic strains, the report said. “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 the emergence of the virus in the human population, regardless of when the initial introduction occurred,” said the NAS report.
“The second wave of the Spanish flu in 1918 was devastating for India, and even if temperature and humidity have some affect on transmission, it may not be as apparent because most people have no immunity to Sars-CoV-2, unlike pre-existing respiratory viruses against which is there is some degree of immunity,” said Dr Dhara.
“In such a scenario, I would suggest a staggered exit to the lockdown where some people can return to work to restart economic activity because a hungry and malnourished population would be more susceptible to infections, including Covid-19,” added Dr Dhara.