Geneva/Washington, March 25, 2020 (WHO/PAHO)- As the COVID-19 outbreak continues to evolve, comparisons have been drawn to influenza. Both cause respiratory disease, yet there are important differences between the two viruses and how they spread. This has important implications for public health measures that should be implemented to respond to each virus.
Q. How are COVID-19 and influenza viruses similar?
Firstly, COVID-19 and influenza viruses are similar. They both cause respiratory disease, which can be asymptomatic or mild but can also cause severe disease and death. Secondly, both viruses are transmitted by contact or droplets. As a result, the same public health measures, such as hand hygiene and good respiratory etiquette (coughing into your elbow or into a tissue and immediately disposing of the tissue), are important actions that everyone can take to prevent both infections.
Q. How fast are COVID-19 and influenza viruses transmitted?
The speed of transmission is an important difference between the two viruses. Influenza has a shorter incubation period (the time from infection to appearance of symptoms) and a shorter serial interval (the time between successive cases) than COVID-19 virus. The serial interval for COVID-19 virus is estimated to be 5-6 days, while for influenza virus, the serial interval is 3 days. This means that influenza can spread faster than COVID-19.
Further, transmission in the first 3-5 days of illness, or potentially pre-symptomatic transmission –transmission of the virus before the appearance of symptoms – is a major driver of transmission for influenza. In contrast, while we are learning that there are people who can shed COVID-19 virus 24-48 hours prior to symptom onset, at present, this does not appear to be a major driver of transmission.
The number of secondary infections generated from one infected individual – is understood to be between 2 and 2.5 for COVID-19 virus, higher than for influenza. However, estimates for both COVID-19 and influenza viruses are very context and time-specific, making direct comparisons more difficult.
Q. How does COVID-19 and influenza affect children?
Children are important drivers of influenza virus transmission in the community. For COVID-19 virus, initial data indicates that children are less affected than adults and that clinical attack rates in the 0-19 age group are low. Further preliminary data from household transmission studies in China suggest that children are infected from adults, rather than vice versa.
Q. What are the differences in symptoms between COVID-19 and influenza?
While symptoms of the two viruses are similar, the percentage of people with severe disease appear to be different. For COVID-19, current data suggests that 80% of infections are mild or asymptomatic, 15% are severe, requiring oxygen, and 5% are critical, requiring ventilation. These percentages of severe and critical infections are higher than with influenza.
Q. Who is most at risk?
Those most at risk of severe influenza infection are children, pregnant women, the elderly, and those with underlying medical conditions and who are immunosuppressed. For COVID-19, our current understanding is that age and underlying conditions increase the risk of severe infection.
Q. Is the mortality rate higher for COVID-19 than for influenza?
Mortality for COVID-19 appears higher than for influenza, especially seasonal influenza. While the true mortality will take some time to fully understand, existing data suggest that the mortality ratio (the number of reported deaths divided by the number of reported cases) is between 3-4%. For seasonal influenza, mortality is usually well below 0.1%. However, mortality is largely determined by access to and quality of health care.
Q. What medical interventions are available for COVID-19 and influenza?
While there are a number of clinical trials currently underway in China and more than 20 vaccines in development for COVID-19, there are currently no licensed vaccines or treatment for COVID-19. However, for influenza, there are a number of available antivirals and vaccines. While the influenza vaccine is not effective against COVID-19 virus, it is highly recommended to get vaccinated each year to prevent influenza.