Although some pundits have suggested that the COVID-19 pandemic will dissipate with coming warm temperatures and high humidity in the Northern Hemisphere, the virus is unlikely to be seasonal in nature, according to a paper published yesterday by the National Academy of Sciences, Engineering, and Medicine.
Summer temps don't mean slower disease spread
In the paper, the National Academies' Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats said that the number of well-controlled studies showing reduced survival of the coronavirus in elevated temperatures and humidity is small and urged caution not to over-interpret these results because of varied and questionable data quality.
Even if warmth were unfavorable for COVID-19, "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," they wrote. "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."
They added that neither the coronaviruses that cause severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) nor the flu strains of previous pandemics have shown a seasonal pattern.
"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," they said. "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."
Studies' short span, assumptions hinder generalizability
Cautioning about the difficulty of determining within-region differences in seasonality because the pandemic started only 4 months ago in winter, mainly in northern latitudes, they cited a number of studies on the topic.
An early Chinese study suggesting that, for every 1°C rise in temperature, daily coronavirus cases decreased by 36% to 57% when relative humidity was 67% to 85.5% and that, for every 1% increase in relative humidity, daily cases decreased by 11% to 22% when the average temperature was about 5°C to 8.2°C (41°F to 47°F). "But these findings were not consistent across mainland China," they said.
Another Chinese study found that rising temperatures and humidity can slow coronavirus reproduction but identified an R0 of nearly 2, suggesting that it is still highly contagious under these conditions. (The R0 [R-naught] is a reflection of how many people each infected person will infect.)
Also in China, research demonstrated sustained coronavirus transmission despite changing weather conditions in different parts of the country that ranged from cold and dry to warm and humid.
A study of 121 countries and areas showed that case growth rates were highest in temperate regions and that rates of growth peaked in areas with a mean temperature of 41°F and decreased in warmer and colder climates. Another study of 310 regions in 116 countries also found an inverse relationship between humidity and temperature and coronavirus incidence.
A Hong Kong study found that, in a suspension of COVID-19 in virus transport medium at 39.2°F, there was only a 0.6-log unit reduction after 14 days. At 71.6°F, there was a 3-log unit reduction after 7 days, and the virus was undetectable after 14 days. At 98.6°F, there was a 3-log unit reduction after 1 day and no virus detection thereafter.
The authors also discussed preliminary findings of laboratory experiments at the Infectious Disease Aerobiology program at Tulane University's National Primate Research Center in New Orleans that found that COVID-19 persists in aerosol at about 68°F and 50% humidity longer than flu virus, the virus that causes SARS, monkeypox virus, and the bacterium that causes tuberculosis.
Differences in real-world, lab conditions
The authors said it is difficult to mesh findings from experimental laboratory studies, which can control some environmental conditions (eg, humidity) but usually don't reflect the real world, and natural history studies, which reflect the real world but can't control environmental conditions and have other confounding factors.
For example, coronavirus transmitted from naturally infected humans to the environment likely has different survival properties than virus grown in the tissue culture media used in many experimental survival studies, they said.
They called for studies of virus-spiked saliva, nasal and lower respiratory tract airway secretions, urine, blood, feces, and nebulized saline. The possibility of differences in environmental viability of different COVID-19 strains should be studied via isolates from early and later in the pandemic and from different geographic areas, they added.