Winter resurgence of Covid-19 predicted as experts warn of hard times ahead

There is growing evidence that seasonal factors could influence the evolution of the current Covid‐19 pandemic, with experts predicting human-to-human transmission of the virus will become more widespread in winter.

The science comes from climatic, behavioural, medical and historic sources and, unfortunately, most point to the same conclusion: we face a long hard winter ahead.

The latest study, published on Tuesday in Transboundary and Emerging Diseases journal, found an association between low relative humidity and an increase in community transmission of Sars-CoV-2 in the Greater Sydney area during the early stages of the pandemic. It estimated that for every one per cent drop in relative humidity, confirmed Covid-19 cases increased by seven to eight per cent.

Although it rains more in winter, the air comes dryer because cold air holds less moisture. It’s why we have “muggy” summers and “crisp” winters. Indoor heating dries things further through evaporation.

Why would the virus prefer dry air? According to Professor Michael Ward, an epidemiologist at the University of Sydney and lead author of the study, it’s because the droplets we exhale become smaller when they have less moisture to become enveloped in.

“When you sneeze and cough, those smaller infectious aerosols can stay suspended in the air for longer,” he told the Australian, the newspaper which first reported the research. “That increases the exposure for other people. When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker,” he said.

A study published by the the Journal of the American Medical Association in June supports the hypothesis that the virus is susceptible to climatic conditions.

It looked at 50 cities and found that those with substantial community spread of Covid-19 were distributed along a narrow band of latitude (30° N to 50° N) with consistently similar mean temperatures (five to 11C) and low specific and absolute humidity. The results were “consistent with the behavior of a seasonal respiratory virus”, the authors concluded.

History suggests we should not be surprised if Sars-CoV-2 exhibits a distinct seasonality. Many past respiratory pandemics, including the mild 2008/9 Swine flu pandemic and the devastating 1918/9 Spanish flu, moved in waves. The same is true of virtually all the 200-plus common respiratory viruses in circulation, including the four coronaviruses that cause the common cold.

It may be tempting to point to places that buck the trend - Dehli for, example. But to say a virus is responsive to seasonal conditions is not to say it can’t spread outside of those time frames. This is especially true when a virus is new and no one has resistance to it (see video above).

“The fact that a virus is seasonal does not mean that it cannot spread at any time of year when conditions are otherwise suitable,” said the epidemiologist Prof Francois Balloux of University College London. “In the presence of a large reservoir of susceptible conditions [and people], Sars-CoV-2 can cause epidemics irrespective of climatic conditions”.

Human behaviour also plays an important role. As the biologist and physicist Richard Neher at the University of Basel has noted, “people spend more time indoors” in winter. 

Combine this with drier, less ventilated rooms and the pronounced seasonality of other coronaviruses and it is clear the European winter poses a clear risk of second spike.

Europe and northern America saw big winter/spring outbreaks. Social distancing, and perhaps the spring, helped contain them, followed by a fairly quiet summer. The southern US states had a broad peak in summer (where people head into air conditioned spaces) and South America and Australia have had large outbreaks in their winter.

“This is consistent with the pattern of the 2009 H1N1 pandemic given it started 4 months later,” Dr Neher wrote on Twitter. 

“To me, this suggests controlling Sars-CoV-2 in the Northern Hemisphere will become a lot harder over the next six months and things might spiral out of control quickly.

“We understand much better now what settings account for most transmission, so we can hopefully contain it without drastic restrictions but it probably won't be as easy as in summer. We need to act early and should head into winter with as few cases as possible,” he added.

This, unfortunately, may be easier said than done. The lifting of lockdown measures, while essential to get economies moving, has triggered a resurgence of Covid-19 cases across Europe over the last month.

Data on cases per head of the population to August 12 from the European Centre for Disease Prevention and Control (ECDC) is still far below the April peak but the trend is up.  Hospitalisations are climbing in Bulgaria, Poland, Romania, Slovakia and Slovenia, and deaths are up in Bulgaria, Luxembourg and Romania.

There is some comfort to be taken from the fact that deaths have not yet moved up in line with confirmed cases in Europe. This may be explained by improving treatments, a higher incidence of the disease in the young and because of “mortality displacement” whereby many of the most vulnerable have already died.

But there is one big unknown: how Sars-Cov-2 will interact with the other seasonal respiratory viruses in circulation this winter. Prof Balloux says this could go one of two ways.

On the bright side, competition with other viruses could see Sars-CoV-2 pushed to the sidelines this winter, either directly or through cross-immunisation via the common cold.

But it could go the other way too. “One - entirely hypothetical - but worrisome scenario would be for co-infection with the flu to lead to particularly severe [outcomes]. A bad flu season, on top of Covid-19 would also challenge healthcare capacity”. 

It is for that reason that the UK government is running its biggest ever flu vaccination programme this year - and is urging everyone to take part. 

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