Summer hasn't stopped coronavirus from spreading, here's why heat can't save us from the pandemic
The effect of weather on the coronavirus has been the subject of extensive debate for several months. There were speculations and many were hopeful that summer would slow down the spread of SARS-CoV-2, the virus that causes Covid-19. Several people, including US President Donald Trump, had suggested that Covid-19 will go away on its own in the warmer weather. However, coronavirus cases in many countries are seeing an alarming rise and it is spreading in many parts of the world where it is hot. So what did some of the studies predict and why did they not come true?
Some suggested heat may slow down Covid-19
Coronaviruses are known to spread more quickly in the winter because they can survive for long in a colder climate. A warm, humid weather makes it harder for respiratory droplets to spread viruses, which is why people speculated that Covid-19 may disappear or at least slow down in summer. The speculations were also based on how seasonal flu viruses behave. They are detected year-round in the US, but flu viruses are most common during fall and winter.
Marc Lipsitch, Professor of Epidemiology and Director, Center for Communicable Disease Dynamics, Harvard TH Chan School of Public Health, addressed the myth that the “common cold coronaviruses are seasonal, with little transmission in the summer, so SARS-CoV-2 will be too". "Predicting how a novel virus will behave based on how others behave is always speculative, but sometimes we have to do so when we have little else to go on. So the first problem with this myth is that we don’t know whether those coronaviruses, which go by the evocative names like OC43, HKU1, 229E and NL63, are good analogies for this virus. The other reason this is a myth is that seasonal viruses that have been in the population for a long time (like OC43 and HKU1) behave differently from viruses that are newly introduced into the population," he wrote.
A preprint posted in March by researchers from Spain and Finland found that 95% of positive cases globally had so far occurred at temperatures between about 28° and 50°F, and in dry climates. Another, by a team led by researchers from Beihang University in China, specifically studied transmission rates across Chinese cities. It found that in the early days of the outbreak, before any government interventions, hot and humid cities saw a slower rate of spread than cold and dry ones.
In a March study, two MIT researchers said that the maximum number of coronavirus transmissions occurred in regions that had temperatures between 3 and 13°C during the outbreak. In contrast, countries with mean temperatures above 18°C have seen fewer than 5% of total cases.
Trump suggested in February that a lot of people think the virus will go away in April with the heat. In April, Trump touted an ‘emerging’ study that indicated sunlight and humidity can weaken the new coronavirus. The federal study said laboratory results showed that increases in temperature, humidity and sunlight can all speed up how fast the virus is destroyed, based on measurements of its half-life when exposed to these elements. The half-life is a measurement of the time it takes for a given amount of the virus to become reduced by half.
In June, a study said that while the rate of Covid-19 incidence does decrease with warmer temperatures up until 52°F, further warmer temperatures do not decrease disease transmission significantly. The research team analyzed daily reported cases of SARS-CoV-2 infection across the US from January 22 through April 3. They said the lowest rate of new cases was observed on days where the temperature was above 50°F five days earlier. The highest increase in infection rates was detected on days when the maximum temperature was below 30°F.
Not climate, public health practices and immunity are key
Experts say the most important reason to be concerned about the ongoing Covid-19 spread is the fact that this is a brand new virus for humans, so almost everyone is susceptible to being infected. Other influences on infection rates include individual behaviors, cultural practices, income and living conditions. Public health practices such as social distancing, the intensity of testing for infection, contact tracing, quarantine of people who are exposed and isolation of people who are infected also play a big role in how the coronavirus spreads, say experts.
"Too many countries are headed in the wrong direction. If populations do not follow the basic public health principles of physical distancing, hand washing, wearing masks, coughing etiquette, and staying at home when sick, if the basics aren’t followed, there is only one way this pandemic is going to go. It’s going to get worse and worse and worse," warned WHO Director-General Dr Tedros Adhanom Ghebreyesus recently.
Immunity to Covid-19 in the population is still low, giving the virus lots of opportunities to spread, say experts. One research, for example, found that humans' current lack of immunity to SARS-CoV-2 — not the weather — will likely be a primary factor driving the continued, rapid spread of the novel coronavirus this summer and into the fall. The researchers developed a mathematical model to simulate how seasonal changes in temperature might influence the trajectory of Covid-19 in cities around the world.
They ran three different scenarios based on what is known about the role of climate in the spread of other viruses, including two coronaviruses called OC43 and HKU1, which are known to cause common colds in people. In all three scenarios, their models showed that climate would only become an important seasonal factor in controlling Covid-19 once a large proportion of people within a given community are immune or resistant to infection. The team said that even if one assumes that SARS-CoV-2 is as sensitive to climate as other seasonal viruses, summer heat would still not be enough of a mitigator right now to slow its initial, rapid spread through the human population. Over the longer term, as more people develop immunity, the researchers suggest that Covid-19 may fall into a seasonal pattern similar to those seen with diseases caused by other coronaviruses.
According to Professor Lipsitch, even seasonal infections can happen "out of season" when they are new. "New viruses have a temporary but important advantage – few or no individuals in the population are immune to them. Old viruses, which have been in the population for longer, operate on a thinner margin — most individuals are immune, and they have to make do with transmitting among the few who aren’t. In simple terms, viruses that have been around for a long time can make a living — spread through the population — only when the conditions are the most favorable, in this case in winter. The consequence is that new viruses — like pandemic influenza — can spread outside the normal season for their longer-established cousins,” he explained.
Dismissing speculations, the WHO had warned in March itself that Covid-19 can be transmitted in all areas, including hot and humid weather. WHO said there is also no reason to believe that cold weather can kill the new coronavirus or other diseases. "The normal human body temperature remains around 36.5°C to 37°C, regardless of the external temperature or weather," it said
In April, experts from the National Academies of Sciences (NAS) also warned that the coronavirus is unlikely to go away once the weather warms up. "Given the lack of host immunity globally, this reduction in transmission efficiency may not lead to a significant reduction in disease spread without the adoption of major public health interventions,” the NAS stated in a letter to the White House.
Many other factors besides environmental temperature, humidity and survival of the virus outside of the host influence and determine transmission rates among humans in the real world, they argued. The researchers said that the other coronaviruses causing potentially serious human illness, including both SARS-CoV and MERS-CoV, have not demonstrated any evidence of seasonality following their emergence. They wrote that pandemic influenza strains have not shown the typical seasonal pattern of endemic or epidemic strains. "There have been 10 influenza pandemics in the past 250-plus years. Two started in the northern hemisphere winter, three in spring, two in 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 researchers.
In one study, experts found that temperature and latitude do not appear to be associated with the spread of coronavirus, but that school closures and other public health measures are having a positive effect. "Summer is not going to make this go away. It's important people know that,” said Prof. Dionne Gesink, a coauthor, and epidemiologist at Dalla Lana School of Public Health.
Another study suggested that warmer and more humid times of the year and locations may offer a modest reduction in Covid-19 reproductive number, helping with efforts to contain the pandemic and build response capacity. But the estimated effects of summer weather are not strong enough to seasonally control the epidemic in most locations, said experts.
In a correspondence published in The Lancet, researchers argued the ability of SARS-CoV-2 to effectively spread globally, suggests that seasonality cannot be considered a key modulating factor of SARS-CoV-2 transmissibility. They said while warmer weather might slightly reduce transmission of SARS-CoV-2, no evidence has suggested that warmer conditions will reduce the effectiveness of SARS-CoV-2 transmission to an extent that few additional interventions are needed to curb its spread. “At present, policymakers must focus on reducing physical contact within communities and any Covid-19 risk predictions based on climate information alone should be interpreted with caution,” warned authors.