Similarities and differences between COVID-19 and influenza – PAHO / WHO

Geneva / Washington, March 25, 2020 (WHO / PAHO) – As the coronavirus disease 2019 (COVID-19) outbreak continues to evolve, comparisons have been made to seasonal flu or influenza. Both cause respiratory illnesses, however there are important differences between the two viruses and how they spread. This has important implications for the public health measures that must be put in place to respond to each virus.

Q. How are COVID-19 and influenza viruses alike?

First, the COVID-19 and influenza viruses have very similar clinical presentations. Both cause respiratory illnesses, with a wide variety of cases ranging from mild or asymptomatic conditions to serious illness and death.

Second, both viruses are transmitted by contact, droplets and fomites. As a result, the same public health measures, such as hand hygiene and good respiratory behavior (coughing into the crook of the elbow or into a tissue and discarding it immediately), are important actions that can be taken to prevent both infections.

Q. How fast are COVID-19 and flu viruses spread?

Transmission speed makes an important difference between the two viruses. The influenza virus has a shorter average incubation period (the time from infection to the onset of symptoms) and a shorter series interval (the time between successive cases) than the COVID-19 virus . The serial interval for the COVID-19 virus is estimated to be between 5 and 6 days, while for the influenza virus it is 3 days. This means that the flu can spread faster than COVID-19.

In addition, transmission in the first 3 to 5 days of illness, or potentially presymptomatic transmission – transmission of the virus before the onset of symptoms – is an important factor in the transmission of influenza. On the contrary, although we are verifying that there are people who can transmit the COVID-19 virus in the 24 or 48 hours prior to the appearance of symptoms, it does not seem for now that it is a significant transmission factor.

The number of secondary infections generated from an infected individual is estimated to be between 2 and 2.5 for COVID-19, higher than for influenza. However, the estimates for both COVID-19 and flu viruses are very context and time-period specific, making direct comparisons difficult.

Q. How do COVID-19 and influenza affect children?

Children are major drivers of flu transmission in the community. For COVID-19, early data indicates that children are less affected than adults and that clinical attack rates in the 0-19 age group are low. Other preliminary data from home transmission studies in China suggest that children become infected from adults, rather than the other way around.

Q. What are the differences in symptoms between COVID-19 and influenza?

While the range of symptoms for both viruses is similar, the proportion of patients with severe conditions appears to vary. In the case of COVID-19, the data collected to date suggests that 80% of infections are mild or asymptomatic, 15% are serious infections, requiring oxygen, and 5% are critical infections, requiring ventilation . These fractions of severe and critical infection seem higher than those observed in the case of influenza infection.

Q. Who is most at risk?

Those most at risk for a serious flu infection are children, pregnant women, the elderly, people with underlying chronic conditions, and people who are immunosuppressed. In the case of COVID-19, we currently believe that advanced age and underlying conditions increase the risk of serious infection.

Q. What disease is the most deadly?

The death rate for COVID-19 appears higher than that of the flu, especially seasonal flu. Although it will take time to accurately determine the true death rate of COVID-19, the data collected so far indicates that the crude death rate (the number of reported deaths divided by the number of reported cases) ranges from 3 % and 4%, although the death rate from infection (the number of reported deaths divided by the number of infections) will be lower. In the case of seasonal flu, the mortality rate is usually well below 0.1%. However, the mortality rate is highly dependent on access to and quality of health care.

Q. What medical interventions are available to combat COVID-19 and the flu?

There are a number of therapies currently undergoing clinical trials in China and more than 20 vaccines under development against COVID-19, but no licensed vaccines or therapies for the novel coronavirus are available at this time. Rather, there are antivirals and vaccines available for the flu. Although the flu vaccine is not effective against the COVID-19 virus, it is strongly recommended to get vaccinated every year to prevent flu infection.

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