Comparing COVID-19 with the flu: More differences than similarities

Photo of Eduardo Sanchez, M.D., M.P.H.Eduardo Sanchez, M.D., M.P.H., Chief, Center for Health Metrics and Evaluation

Many have compared seasonal influenza (the flu) with COVID-19, the disease caused by SARS-CoV-2. This is understandable, to some degree, because to understand the new or unknown we sometimes compare it with something known or familiar and seek similarities between the two. For example, the flu and COVID-19 are both contagious respiratory diseases caused by viruses, and they share some of the same symptoms (e.g., fever, cough) and prevention tactics (e.g., handwashing, staying home when sick).

But based on our current understanding, there are several reasons why COVID-19 is not “just like the flu”:

  1. SARS-CoV-2 is a novel virus. Unlike the influenza A and influenza B viruses responsible for annual seasonal flu epidemics, the virus that causes COVID-19 has never been encountered by the human body. Scientists are working quickly to understand the molecular structure of the virus and the mechanisms of how it causes various symptoms in humans, but much remains unknown.
  2. There is no widespread immunity to COVID-19. As far as we can tell, anyone who has not had COVID-19 (with or without obvious disease) has the potential to become infected with COVID-19. It is a new disease and low percentages of people in communities have built immunity to it. Furthermore, our understanding is limited with regard to the timeline and the extent to which people could become re-infected with COVID-19 (if at all), but we’ll learn more as the pandemic progresses and data accumulate.
  3. There is no vaccine for COVID-19. Vaccines stimulate a person’s immune system to produce immunity to a specific disease, which helps protects the person from that disease. The flu vaccine can prevent a person from getting sick with the flu, reduce the risk of flu-associated hospitalization, and reduce the severity and duration of illness in people who still get sick. But with COVID-19, no vaccine exists. The race is on, but vaccines typically take months to develop, test, distribute and administer to the population.
  4. There is a lack of consistently effective treatment for COVID-19. The U.S. Food and Drug Administration (FDA) has approved several treatments for the flu that have been evaluated and are widely used. On May 1, FDA issued an emergency use authorization for the investigational antiviral drug remdesivir for the treatment of COVID-19 in adults and children hospitalized with severe disease, but information about the safety and effectiveness of the drug for this use is limited. Moreover, there are no known treatments for people with less severe disease, let alone to prevent or cure the disease.
  5. COVID-19 seems to be more contagious. Both diseases spread from person to person through respiratory droplets produced when a sick person coughs or sneezes. The ease with which a particular virus spreads is characterized by estimating the average number of people who catch the virus from a single infected person. Available information suggests that SARS-CoV-2 is spreading more efficiently than influenza. It spreads easily and sustainably, meaning that it goes from person to person without stopping.
  6. COVID-19 has claimed more lives in the United States this year to date than seasonal influenza does in a typical year. CDC estimates that 24,000 to 62,000 flu deaths occurred during the 2019-2020 flu season (October 1, 2019 through April 4, 2020). This is a wide range because flu is not a reportable disease in most areas of the country, and the estimate is derived from a mathematical model based on flu-associated hospitalizations. As of May 14, 2020, CDC showed 83,947 COVID-19 deaths (the first of which were reported in February 2020), which includes deaths counted and reported daily by state, local and territorial health departments. It’s notable that these COVID-19 deaths were reported during a time period that was approximately half as long as the flu season.

    A line graph displaying the COVID-19 and Influenza Deaths in the U.S. by Month (February - April 2020)

    Click on graph to enlarge.

    The line graph compares monthly deaths from COVID-19 and from influenza during February, March and April 2020. The numbers illustrated in the graph are based on CDC data indicating the total number of deaths received and coded through early May. These provisional death counts are based on incoming data from death certificates. Deaths are coded as COVID-19 when it is reported as a cause of death or when it is listed as a “probable” or “presumed” cause, which can include cases with or without laboratory confirmation. These data represent the most accurate death counts but have real-time differences from CDC’s reported number of total COVID-19 deaths in the United States because it can take several weeks for death certificates to be submitted and processed.

    But whether one uses estimated or counted deaths from the flu, the data indicate that already, COVID-19 has been associated with more deaths in 2020 than the flu in this most recent (2019-2020) influenza season.

Mortality rate comparisons need context

The comparisons that have been drawn between U.S. mortality rates of COVID-19 and the flu do not always include critical context.

The mortality rate for the flu is believed to be around 0.1%. As described above, this is based on surveillance data, not on documentation of every person who is infected with and dies from the flu.

The U.S. mortality rate for COVID-19 at this time is approximately 6.0%, based on CDC’s number of reported deaths divided by reported cases as of May 14, 2020. This is a crude mortality rate (also known as case fatality rate); the true mortality rate (also known as infection mortality rate) is likely lower because it appears that many people who have COVID-19 are asymptomatic or mildly symptomatic, are often not tested, and therefore are not counted as a case. Though asymptomatic people are less likely to die from COVID-19, they can transmit it to other people—and could be more likely to do so because they are unaware that they are infectious.

The key point is that mortality rate comparisons should be interpreted with caution, especially as data on reported COVID-19 cases and deaths are evolving daily.

Surveillance is critical

Public health surveillance is “the ongoing, systematic collection, analysis, and interpretation of health-related data essential to planning, implementation, and evaluation of public health practice.” Our response to the COVID-19 pandemic can only be as good as our surveillance. Information about measures such as the disease’s prevalence, severity and death rate is critical for decision-making about public health and health care containment and mitigation efforts. Surveillance measures that are stratified by age, sex, racial/ethnic group, presence of underlying conditions, or other categories help identify groups of people or types of conditions that are associated with higher risk for certain outcomes, such a mild, severe or fatal COVID-19.

More and better local data yields better understanding. National and statewide estimates are important, but they can mask important differences between counties and other jurisdictions. Communities need local data to guide their decisions about enacting preventive measures, enabling rapid response and containment strategies, preparing health care systems for potential cases and resuming activities following shelter-in-place orders.

A final word

Our understanding of the virus is rapidly evolving, but so far, COVID-19 appears to pose a greater risk to the public’s health than the flu. As we learn more about SARS-CoV-2 and myriad effects of COVID-19, our strategies to help prevent the virus’ spread and mitigate its impact will evolve. And until effective treatments and vaccines are universally available, the only protections are prevention and rapid containment.