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Influenza and COVID-19

There is significant confusion when it comes to comparing the seasonal flu caused by the influenza family of viruses, and COVID-19 caused by the “novel” coronavirus. The seasonal flu occurs every year. In the US, it begins in the fall and reaches its peak around February, then begins to taper until activity is gone for the summer. Complications of the flu virus are numerous, but the one that results in the most deaths is pneumonia. This is an infection of the lungs that can be caused by the flu virus directly, or can occur as a result of the flu virus creating ideal conditions for bacteria to infect the lungs.

The US National Center for Health Statistics (NCHS) is housed within the Center for Disease Control (CDC). It tracks deaths related to influenza and reports them in several ways. The most often cited numbers from the CDC include deaths from influenza and pneumonia combined. However, the NCHS also reports these deaths separately. The chart below shows the NCHS data from 2013 to 2020 for influenza and pneumonia deaths individually.

This year, a global pandemic is unfolding due to a novel (new) coronavirus and its corresponding illness COVID-19. Like other respiratory viruses, it affects the lungs. Death can occur as a direct result of the virus or, similar to the flu, it can occur as a result of a complication like bacterial pneumonia. The chart above also shows data collected by the NCHS for COVID-19.

Examination of the trends in the graph leads to several questions:

1.Is there a relationship between influenza (flu) and pneumonia?

Yes. The two are intimately related. As cases of influenza increase seasonally, so do hospitalizations from pneumonia. The graphs shows the two kinds of deaths increasing and decreasing together every season. It also shows that influenza cases practically disappear each summer, but pneumonia cases do not. Though the two are related, influenza is not the sole cause of pneumonia.

2.Why does pneumonia result in so many deaths?

Pneumonia can be caused by viral, bacterial, or fungal infections. People with multiple medical problems, a history of lung disease, or problems of the immune system are more susceptible to pneumonia, and more likely to have a worse outcome. Those who require hospitalization have longer hospital stays, and those who require intensive care or breathing support (ventilator) have a higher risk of death. Even with the best treatment, critical illness from pneumonia can lead to other body systems failing, further complicating matters. It remains a dangerous disease for those patients with multiple risk factors.

3.How many pneumonia cases are the result of an influenza infection?

This number is difficult to calculate. Influenza can injure the lungs directly, causing a pneumonia. However, influenza can also create conditions favorable for bacterial pneumonia. In addition, a bacterial pneumonia can occur at the end of an influenza infection when the virus is no longer detectable. In those cases it is difficult to tell if influenza was the cause since the virus has been cleared by the body. The chart above tracks deaths due to influenza and pneumonia separately. Although this does not answer the question, it does demonstrate a clear relationship between the two.

4.How did COVID-19 effect the US early in 2020?

The graph shows a sudden, large spike in COVID-19 deaths starting in the 12th week of 2020. This peaks around week 15 and then begins to fall.

5.Is there a relationship between COVID-19 and pneumonia?

Yes. The graph shows an equal increase in pneumonia deaths occurring simultaneously with COVID-19. Additionally, we can see that influenza deaths continued to decrease as they have done in prior seasons. We can conclude that there is a relationship between COVID-19 and pneumonia, similar to influenza. This is not surprising because both the novel coronavirus and influenza are respiratory viruses, primarily attacking the lungs. The complication of pneumonia is expected.

6.Can we say that COVID-19 is like the flu?

This is a complicated question. Both are respiratory viruses that can lead to death by direct lung injury or pneumonia. However, the graph above shows a very rapid increase in death from COVID-19. The peak is well above the expected number of pneumonia or flu deaths, making COVID-19 far worse. In fact, most of the COVID-19 deaths reported come from outbreaks in a small number of large urban cities. The remainder of the US has not experienced the number of cases that New York City, New Orleans, and others have seen. When you take that into account, it becomes clear that COVID-19 can cause a rapid and overwhelming presentation of critically ill people who die from the virus or subsequent complications. Such surges of patients in need of significant resources can quickly overwhelm any hospital. This is the primary reason for social distancing. (Read more here “Is It Possible To Avoid Coronavirus“)

7.Is the NCHS data complete?

No. The CDC website states that there can be a lag of 1-8 weeks before a state reports a death to the NCHS. That means the numbers for the past 8 weeks may actually be higher. That also means that the COVID-19 effect may be worse than what is displayed in this graph.

8.Are all the deaths listed for COVID-19 occurring in people who tested positive?

No. The CDC website also states that this number includes “Deaths with confirmed or presumed COVID-19, coded to ICD–10 code U07.1” This means that a patient is diagnosed with COVID-19 by a physician utilizing this diagnosis code on their billing records. It is not tied to a specific COVID-19 test. This is important, especially in a pandemic or large outbreak, because diagnosis of a disease can be made clinically, without a specific COVID-19 test. Early on in the US experience, COVID-19 testing was not available. This does not mean there were no cases, simply that cases were diagnosed based on other criteria.

9.Do the reported pneumonia deaths include people who tested positive for the flu?

No. Once again, the CDC website notes “Pneumonia death counts exclude pneumonia deaths involving influenza.” Patients who tested positive for influenza and died are reported separately.

10.What can we conclude from this “preliminary” data ?

We can see that a major event has occurred with the arrival of the novel coronavirus in the US. The number of deaths due to COVID-19 rapidly increased and far exceeded prior influenza experience. With it, came a large increase in pneumonia deaths demonstrating that influenza is not the only respiratory virus which can lead to significant numbers of pneumonia hospitalizations and death. Although the US healthcare system prepares every year for the seasonal flu and the accompanying increase in critically ill patients, the COVID-19 outbreak was not something it was prepared to handle. Also, the arrival of COVID-19, while at the end of the influenza season, brought far more critical illness and death in such a short time frame, than healthcare systems have seen in recent history.

As we begin to ease restrictions originally put into place to prevent overwhelming medical resources, these insights can help inform discussion and decisions.


  1. CDC Weekly US Influenza Surveillance (Pneumonia and Influenza (P&I) Mortality Surveillance)
  2. CDC Provisional Death Counts for Coronavirus Disease (COVID-19)

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