Why are there flu seasons and could COVID-19 follow the same pattern?

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At peak flu season five million people worldwide are infected by the virus causing a quarter of a million deaths. Increasing numbers of influenza virus (flu) cases from summer to winter creates huge demand on health services and remains a burden despite the existence of a flu vaccine. But what’s so special about winter for the spread of the contagious influenza virus, and could this mean the same fate awaits the recently emerged SARS-CoV-2 coronavirus?

There are two main types of flu that circulate in the human population causing seasonal pandemics: influenza A and influenza B. However, within these two strains there are many different subtypes and subclades creating a large pool of different variations of influenza strains that can infect people.

The strains can mutate randomly at various rates to create new strains that are genetically different to previous ones. These have potential to cause pandemics because there will be no previous immunity in the population, thus these outbreaks result in many infections which can become a pandemic.

An increase in flu infections in winter is actually a result of human behaviour

Despite the huge number of circulating flu strains a vaccine is made available annually in autumn to those who the virus has the most drastic effect on: the elderly, very young, and immune-compromised people. Due to the unpredictability of the influenza virus, flu vaccines change year on year and different vaccines are available depending on your age and health, however it is difficult to generate protection against all strains with one vaccine, so there is still a risk of infection.

It might seem obvious that the cold is what’s keeping the flu around in the winter months, but what creates the increase in infections is actually more to do with human behaviour. In winter, people spend more time indoors so are more likely to breathe the same air as someone infected by the flu and contract the virus themselves.

Additionally, in winter the air is drier as cold air can carry less water than warm air. Research into viral survival found that these conditions are more suited for influenza to flourish and spread further. Moreover, water in the air can be toxic towards viral particles and may alter acidity or salt concentrations deforming surface proteins on the virus so that entry and infection into humans are inhibited. In drier air, virus particles can stay aloft much longer and cause infection more effectively.

Even when sneezing in drier air the particles break down into smaller pieces and can stay airborne much longer than in moist air where the particles are heavier and drop quicker. This makes conditions in winter ideal for viral transmission and can help explain seasonal flu epidemics.

Other coronaviruses that infect humans also follow seasonal patterns

The immune system has a role to play as well. With more time spent indoors and shorter days with less sunlight, people don’t get as much vitamin D which is a key component of immune responses and helping to keep the body primed against infection. As well as this making us more vulnerable, in the cold, blood vessels tend to constrict to help maintain core body temperature. However, when blood vessels constrict in the nose this could impair immune cells reaching mucus membranes there, allowing viruses to slip past defences and into the body.  

Like flu, four of the seven common coronaviruses that infect people and cause similar respiratory illness follow seasonal patterns of infection. Whilst it is still too soon to say whether SARS-CoV-2 will follow those trends, previous data from longitudinal coronavirus studies suggests that most cases were detected between December and April.

Studies on viral survival from the COVID-19 pandemic have yielded contradictory results with some studies suggesting there is no difference between survival in moist and dry air, and one suggesting that the coronavirus spreads better in warmer air. It is unlikely that SARS-CoV-2 case numbers will decrease over the summer months this year given that many cases have been reported in countries with warmer climates such as Iran and Singapore. 

Eventually, it is believed that SARS-CoV-2 will display seasonal patterns, however currently there are likely too many cases thus increasing the infection rates without any environmental input. If SARS-CoV-2 becomes a seasonal infection it will allow time during the year to prepare, and with over 62 different vaccine trials currently occurring, there is hope for a successful vaccine to also help limit spread.

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