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COVID-19 or the Flu? The WHO Helps You Know the Difference

Sarah R. Scott BeijingUnitedFamilyHospital 2021-01-19

Is it COVID-19 or influenza? Both cause respiratory disease, yet there are important differences between the two viruses and how they spread. The World Health Organization (WHO) responds to some common questions and how to spot the differences between this new coronavirus and the flu.

DISCLAIMER: The situation surrounding the COVID-19 epidemic is rapidly changing. All information in the proceeding article is up-to-date as of the date of publication.


This article was adapted from Q & A: Similarities and differences–COVID-19 and influenza. https://www.who.int/news-room/q-a-detail/q-a-similarities-and-differences-covid-19-and-influenza. 


How are the SARS-CoV-2 and influenza viruses similar?

Firstly, COVID-19 and influenza present similarly. That is, they both cause respiratory disease, resulting in a wide range of illness, from asymptomatic or mild to severe disease or sometimes even death.

 

Secondly, both viruses are transmitted by contact and 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 all can take to prevent infection. 



How are COVID-19 and influenza viruses different?

The speed of transmission is an important point of difference between the two viruses. The flu has a shorter median incubation period (the time from when you get infected to when you begin to show symptoms) and a shorter serial interval (the time between successive cases in a chain of transmission) 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. Though COVID-19 can be transmitted 24-48 hours prior to symptom onset (asymptomatically), at present, this does not appear to be a major driver of transmission

 

The reproductive number, (R0 or R-naught), is the number of secondary infections generated from one infected individual. This is estimated to be between 2 and 2.5 for the SARS-CoV-2 virus, which is higher than for the flu. However, estimates for both COVID-19 and influenza viruses are very context and time-specific, making direct comparisons more difficult.  

 

Children are important drivers of influenza virus transmission in the community. For COVID-19, initial data indicates that children are less affected than adults. Further preliminary data from household transmission studies in China suggest that children are infected from adults, rather than vice versa.

 


While the range of symptoms for the two viruses is similar, the fraction with severe disease appears to be different. For COVID-19, data to date suggest that 80% of infections are mild or asymptomatic, 15% are severe infection, requiring oxygen, and 5% are critical infections, requiring ventilation. Cases of severe and critical COVID-19 infection are higher than what is observed for influenza infection.


Those most at risk for severe influenza infection are children, pregnant women, elderly, those with underlying chronic medical conditions, and those who are immuno suppressed. For COVID-19, our current understanding is that older age and underlying conditions increase the risk for severe infection.



Mortality for COVID-19 appears higher than for influenza, especially seasonal influenza. While the true mortality of COVID-19 will take some time to fully understand, the data we have so far indicate that the crude mortality ratio (the number of reported deaths divided by the reported cases) is between 3-4%, the infection mortality rate (the number of reported deaths divided by the number of infections) will be lower. For seasonal influenza, mortality is usually well below 0.1%. However, mortality is to a large extent determined by access to and quality of health care. 

 

What medical interventions are available for COVID-19 and influenza viruses?

While there are a number of therapies currently in clinical trials in China and more than 20 vaccines in development for COVID-19, there are currently no licensed vaccines or therapeutics for COVID-19. In contrast, antivirals and vaccines are available for influenza. While the influenza vaccine is not effective against COVID-19 virus, it is highly recommended to get vaccinated each year to prevent influenza infection


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