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Complex patterns of spread in respiratory viruses

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How and when is the novel coronavirus, COVID-19, transmitted and what can we expect from this outbreak in the months ahead? Respiratory viruses generally have complex patterns of spread, but there are similarities with earlier known coronaviruses, research from the University of Gothenburg shows.

“We have four coronaviruses that normally circulate in the population. What we’ve seen is that they’re extremely seasonal, peaking in winter. They correlate strongly with low temperatures and low humidity,” says Nicklas Sundell, a recent PhD graduate in infectious diseases at Sahlgrenska Academy and specialist doctor at the Department of Infectious Diseases, Sahlgrenska University Hospital/East.

“Once spring has got going and the temperature’s going up, the virulence of the contagion is reduced,” he continues. ”We can speculate that the new COVID-19 has these properties too, but we can’t say for sure. What’s more, a pandemic is underway and everyone’s susceptible to the virus, so the situation’s different.”

Infections caused by respiratory viruses are very common in humans and, globally, are associated with high morbidity and death rates. The main purpose of the thesis was, from epidemiological, clinical, and diagnostic studies, to obtain in-depth knowledge of viruses that affect the respiratory system and of factors that can influence their spread.

Kick-started by a cold spell

In a substudy that attracted media attention a few years ago, more than 20,000 virus samples from the upper respiratory tract were reviewed over a three-year period. The study shows that the incidence of influenza and most other ”enveloped” respiratory viruses is strongly associated with cold weather, while non-enveloped viruses, such as rhinovirus and enterovirus, cause infections throughout the year.

Increased prevalence of, for example, influenza viruses, coronaviruses and respiratory syncytial virus (RSV) is strongly correlated with low outdoor temperature and low absolute humidity. This may be due to the fact that cold, dry weather contributes to the transmission of these viruses, by favoring the formation of tiny airborne particles (aerosols) that can stay suspended in the air for a longer period of time.

“We also found that peaks of the annual, seasonal flu, at our latitude, coincide with sudden falls in temperature, often below zero degree Celsius. These results are vital to be able to understand the seasonal variation of respiratory viruses and predict the approximate timing of the annual flu season,” Sundell states.

The thesis also includes research on how common respiratory viruses are in asymptomatic (symptom-free) adults. Fewer than one in 20 proved to be hosting a virus. This result provides more knowledge of how to interpret tests and analyses.

How viral respiratory infections can be distinguished from bacterial infections in hospital inpatients was also studied. To avoid unnecessary use of antibiotics, keeping the patients apart is essential. This also applies in purely literal terms to avoid the spread of infection in the hospital environment.

Major differences in transmission

Another part of the thesis, with a bearing on the ongoing pandemic, is a study of the measles outbreak of 2017–2018 in Gothenburg. Those who fell ill despite previous vaccination had milder symptoms and smaller numbers of infectious particles in the nose, and did not transmit the virus to anyone else. People with “primary” measles, on the other hand, had significantly more virus particles in the upper respiratory tract and were affected by coughing more often.

“The results are highly important for managing future measles outbreaks, in terms of contact tracing and preventive measures. Although there’s no study of the novel coronavirus in the thesis, the number of virus particles in the upper respiratory tract are likely a major factor in its transmissibility,” Sundell says.

Title: On the epidemiology, clinical presentation and transmission of respiratory viral infections

Contact: Nicklas Sundell

Images: Genre image from iStock and portrait pivture of Nicklas Sundell