One year after the emergence of the disease that killed 2 million people, humans continue to underestimate the SARS-CoV-2 virus.
This winter, Covid-19 is proving to be even more dangerous than epidemiologists and public health officials feared – and not just because of the more contagious variants that are now circulating around the world. Recently, in October, Nature reported that it was “too early to say whether Covid is seasonal like the flu”. Evidence suggests that winter may increase virus transmission: in the laboratory, the virus persisted in cold, dry conditions and was inactivated by the ultraviolet rays of sunlight.
There were reasons to hope that this was not the case. Coronaviruses, which generally show less seasonal variation than the flu virus, tend to have a weak response to changes in temperature. Outbreaks such as the severe acute respiratory syndrome (SARS-CoV) coronavirus in 2002 were not considered seasonal, although the outbreak ended too quickly for scientists to definitively test this idea.
Still, SARS-CoV-2 appears to be different, and the world may face a bigger fight this winter than expected. “People are thinking they are seeing something like the flu, and it is much worse,” says Professor Richard Carson, who published a preprint in November suggesting that Covid’s reaction to temperature changes, known as its response curve to temperature, can be even more pronounced than the flu, the definitive seasonal virus. “Many of the things people did in the summer that we thought we were working on were actually higher temperatures, giving the impression that these things were working.”
Many of the things people did in the summer that we thought we were working on were actually higher temperatures, giving the impression that these things were working.
Carson and his co-authors from the Massachusetts Institute of Technology and Wake Forest University came to that conclusion by digging into a unique set of data: state death certificate records. Statistics on the case count and deaths of Covid-19 in the USA are notoriously loud. Local and state governments do not report standardized data. Laboratories overloaded with tests can delay the reporting of positive cases long after administering the tests. Deaths reported on any given day may have occurred weeks earlier. All of these inaccurate counts prevent attempts to measure the role of temperature in transmission rates.
But Carson’s team, experienced in applying econometric techniques for modeling and forecasting environmental impacts, found a solution in Massachusetts’ Covid-19 statistics report last year. “Buried at the bottom of the report was an alternative death count on the date of the death certificate,” he said. “Once we found it, it took us two months to get data from most major states.” Taking the dates of death certificates, the team can compare Covid-19’s death trends with maximum daily temperatures over a three-month period between April 16 and July 15.
According to the survey (pdf) now under peer review, the data show that the virulence of the virus increases below 31 ° C (88 ° F). His “sweet spot,” says Carson, is around 4.4 ° C (40 ° F), but that’s just his favorite condition. Temperatures between 5 ° C and 10 ° C (41 ° F to 50 ° F) favor transmission and infection.
The study model revealed a strong correlation between temperature change and the number of cases and deaths of Covid-19 from a baseline of 31°C, the mid-summer average in the USA. When the weather drops to a cold 5 ° C, the model shows a 160% increase in the number of deaths due only to the influence of cold weather, even after controlling for shelter requests on the spot.
The Covid-19 transmissions showed an even stronger effect. Four times more positive cases of Covid-19 are expected when temperatures drop to 5 ° C, assuming that there are no other interventions, such as masks or social detachment. Only when temperatures drop a few degrees below zero, the point at which water droplets quickly freeze in the air, Carson and his team project that the transmission will decrease.
“It’s a really scary newspaper,” says Carson. “We know the temperature response curve for the flu. This is much more steep. “
Carson et. al. 2020
For most of the United States, located in the northern hemisphere, a temperature response curve like this makes winter treacherous. Any delay in responding to the high activity of the virus, the data indicate, will lead to a rapid escalation in cases due to the feedback loop between the cold temperature and the exponential growth curve of the virus. “The patterns of the mid-December outbreak correspond almost exactly to what the temperature response could predict,” says Carson.
Adam Kaplin, a physician and public health researcher at the John Hopkins University School of Medicine, was not surprised. “It’s really hard to see it any other way,” he said, having come to a similar conclusion in his recent survey (pdf) to the newspaper PLOS ONE. “The transmission of the virus is increasing because of the temperature. This is clear. “
The transmission of the virus is increasing because of the temperature. That is clear.
Kaplin analyzed SARS-CoV-2 transmission rates and daily temperature records in 50 countries between January and April 2020, a period before most mask orders went into effect, allowing his international team of researchers to isolate the influence of temperature. They found that for each degree temperatures dropped between 30 ° F and 100 ° F, the transmission rate increased by 3.7%
If this “strong and robust” association is correct, says the newspaper, countries must spend their spring and summer months containing the virus to have any hope of containing winter outbreaks, given the effects of falling temperatures. “This is a race that we should have done a lot earlier,” says Kaplin. “We should be way ahead of that. We spoiled it and many people died who didn’t need to die. “
What is causing this?
Viruses kill millions of people each year, but we know very little about why their virulence fluctuates from season to season. The spread of flu in winter is commonly attributed to patterns of human behavior – staying indoors, where germs spread more easily – but that explanation has been questioned. “I don’t think you’ll find a uniform answer to that question,” says Joe Eisenberg, president and professor of epidemiology at the University of Michigan School of Public Health. “This research has not been prioritized in the past.”
Carson was unable to discover the relative influence of viral biology and human behavior on the SARS-CoV-2 response to low temperatures. His research was only able to eliminate moisture as a probable transmissibility factor; temperature and ultraviolet light (which fluctuates in sync with temperature) were much better at explaining Covid-19’s transmission patterns.
But Kaplin believes the evidence points to a very strong candidate. “It is the biology of the virus”, he argues. “Yes, people are coming in more, but that plays a much smaller role than biology.” Nothing in the scientific literature, he notes, supports the idea that this seasonality is driven by human behavior. In the newspaper Medical Hypotheses, a 2016 article found no evidence that crowding in winter drives seasonal viral transmission, noting that the time spent indoors in the U.S. changes less than 10% between summer and winter. Evidence in the tropics, where the flu circulates all year, also contradicts this hypothesis.
Political implications
Reducing the rate of transmission will mean redoubling the mask and efforts at social distance. Now, preventive measures must increase only to keep the rate of spread of the epidemic under control.
It is a bleak perspective. “The great notion of politics is that every week the temperature drops, you have to reduce the effective contact rate to keep the virus in check and prevent exponential growth,” says Carson. “People are not doing this, which is why you are seeing these outbreaks.”
The great notion of politics is that every week the temperature drops, you have to reduce the effective contact rate to keep the virus under control and prevent exponential growth.
The new variant adds a wildcard to the mix. “This new variant is more transmissible, but it can still have a strong seasonal signal,” says Eisenberg. “We just don’t know.”
So far, lawmakers have waited too long to respond to Covid-19’s rising winter. In the UK, cases started to pile up again this fall, after a brief break in the summer. Hospitals have warned of a fall with the arrival of colder weather. But national restrictions were not reimposed until January 9, when the health care system was already in crisis struggling with the most contagious strain. Today, Covid-19’s per capita death rate in the UK is among the highest in the world and more than 40,000 Covid-19 patients are hospitalized, almost double the peak from last year.
The United States is even worse off. The country has never been able to control the pandemic. Although transmission decreased during the summer, the virus returned with full force. The US now accounts for 20% of Covid-19 deaths worldwide – despite having less than 5% of its population – and is on track to reach 600,000 deaths by 2021. The only immediate solution is to establish herd immunity before the next winter through a mass vaccination campaign that reaches over 70% of the population. If the pandemic continues to get out of hand, many more deaths are inevitable.
But we can prevent this winter from getting any worse, says Eisenberg. The public health measures we have already taken need to be accelerated if we are to keep SARS-CoV-2 under control. “It intensifies our recommendations,” he says, “but it doesn’t change our recommendations.”