The virus responsible for COVID-19 and the flu virus manifests themselves in a similar way. The World Health Organization (WHO) has tried to clarify this in a document, explaining the differences between the two viruses and their spread.

We discussed this with Dr Michele Lagioia, Medical Director of Humanitas.

Both cause respiratory disorders, which can occur in very different ways. Patients can be asymptomatic, have low level symptoms, have more serious pathology symptoms, and it may also cause death.

Influenza and coronavirus share the same method of transmission. Contact, nasal droplets, materials contaminated by the infected person. The hygiene rules that can offer concrete help in limiting their spread are the same: wash your hands often, avoid touching your face, sneeze into the inside of your elbow or into a tissue, which should be thrown away immediately. 

What are the differences between the two viruses?

A first immediate difference is the speed of transmission of the two viruses. 

Influenza has fewer days of incubation (the time between infection and the onset of symptoms) and a shorter serial interval (the time that passes between subsequent cases) and is therefore transmitted more quickly. If the interval of COVID-19 is 5-6 days, that of influenza is 3 days.

In addition, influenza has a higher rate of infection in the first days of illness or even in the pre-symptomatic period. In fact, even if cases of COVID-19, transmission by asymptomatic subjects have been documented (in a period of 24 to 48 hours). At the moment, these subjects are not the main vehicles of the COVID-19 infection.

The number of infections generated by a single case of coronavirus varies between 2 and 2.5, which is higher than that of influenza, however, it should be noted that direct comparisons between influenza and COVID-19 viruses are very difficult. Virus-related estimates are closely related to the context of analysis.

Children, in particular, are an important transmission vehicle for influenza. From preliminary data this seems not to be the case for the coronavirus, as its impact in the 0-19 age group is much lower than in other groups. This does not mean, unfortunately, that children cannot be infected. Preliminary data from studies conducted in China on the subject indicate, however, that sick children have been infected by adults, and not vice versa.

Another difference between the two viruses, this time more obvious, can be found in the percentages of severe forms. As far as COVID-19 is concerned, the data indicates that 80% of cases are of low severity or asymptomatic, 15% are serious infections, 5% are critical cases (although in Italy the percentage of severe cases is unfortunately higher, with a particularly high lethality rate). In comparison, influenza has a lower percentage of critical cases.

The data available to date indicate that the mortality rate (understood as the ratio of the number of deaths to the number of cases) varies between 3 and 4%. The mortality rate from infection (understood as the ratio of reported deaths to the number of reported infections) tends to be lower.

In terms of risks related to the patient’s age and general condition, if the flu is riskier for children, pregnant women, the elderly, immunosuppressed people and those suffering from chronic diseases, the coronavirus seems to cause more damage to the elderly and those with underlying diseases. 

Finally, it should be noted that the effects of influenza can be limited (if not avoided) by a vaccine, something which we do not yet have against the COVID-19 virus.