Influenza is one of the deadliest viruses in the world, yet we take for granted that we are protected from its ill effects. We are concerned about other viral infections, some of which affect far fewer people. Yet many of us skip our annual flu shots, giving various excuses for forgoing the vaccine. The impact of influenza is enormous—about 1 billion cases worldwide with a death rate estimated to be nearly half a million people annually.

Over the last several years in the United States, we have seen moderate to severe flu seasons. During the COVID-19 pandemic, flu cases have subsided, providing evidence for the effectiveness of masks and physical distancing.

The flu seasons leading up to the COVID-19 pandemic were particularly severe. The 2017–2018 influenza season was notable for an unusually long duration of widespread high influenza activity throughout the U.S. and higher rates of outpatient visits and hospitalizations compared with recent seasons. The 2017–2018 season was the first season to be classified as high severity across all age groups (CDC, 2019, September 5).

During the 2017–2018 flu season, vaccination is estimated to have prevented more than 7 million illnesses, nearly 4 million medical visits, more than 100,000 hospitalizations, and 8,000 deaths, despite an overall estimated vaccine effectiveness of 38%.

Grohskopf et al., 2021

The 2018–19 influenza season was of moderate severity and differed from recent seasons in that there were two waves of influenza A activity of similar magnitude. It was also longer than recent seasons with activity at or above baseline for 21 consecutive weeks. Overall, hospitalization rates were below those of the previous season, but rates for children under 17 years of age were similar to the previous year (Xu et al., 2019).

Although influenza activity during the 2020–21 season was low throughout the U.S., influenza vaccination remains an important tool for the prevention of potentially severe respiratory illness, which might decrease stress on the healthcare system during ongoing circulation of SARS-CoV-2 (Grohskopf et al., 2021).

The 2021–22 influenza season has coincided with continued circulation of COVID-19. Influenza vaccination of persons aged ≥6 months will reduce symptoms that might be confused with those of COVID-19. Preventing and reducing the severity of influenza illness continues to be an important factor in reducing stress on the U.S. healthcare system.

The Emergence of COVID-19

Respiratory tract infections are the most common type of infections worldwide, representing a source of significant morbidity and a considerable economic burden. In late 2019, a respiratory virus with symptoms similar to those caused by influenza (SARS-CoV-2, or COVID-19) began to spread globally. Within a short period of time, this highly infectious virus spread to every country in the world. Like flu, SARS-CoV-2 spreads easily via droplets, via direct and indirect contact, and via tiny aerosol droplets that can stay suspending in the air for more than an hour.

Comparison with 1918–1919 Flu Pandemic

The emergence of a global pandemic 100 years after the 1918–1919 influenza “mother of all pandemics” has led us to look back to analyze what worked and what didn't work in 1918. What started as a mild outbreak in the spring of 1918 was followed by a much more serious outbreak in the fall. Scientists and politicians initially downplayed the pandemic, believing it was caused by a bacterium rather than a virus—perhaps cholera or bubonic plague. With no vaccine and few analytic tools, public health practices such as masks, social distancing, refraining from spitting in the street, and covering your mouth when sneezing became important tools to slow the spread of the virus.

Curiously, many of the political and personal issues we face today also occurred in 1918. Early on, public health officials, including President Woodrow Wilson, argued that the flu outbreak was caused by “ordinary” influenza and stated that the public need not worry so long as simple precautions were followed. There was no general lockdown, although many cities closed saloons, theatres, and places of public gathering (Soucheray, 2020, April 10) and, in the end the areas of the country that enforced strict public health measures fared better than areas that did not.

In January 1919, President Wilson and several members of his staff were felled by a severe case of the flu. The White House physician downplayed the President's illness, arguing that his symptoms were due to overwork and rainy weather (Solly, 2020, October 2).

Resistance to mask wearing, quarantine orders, and bans on public gatherings was common in 1918 (many people poked holes in their masks to allow smoking). Despite this, fear was pervasive and there was widespread absenteeism from work, whether out of fear or because workers were caring for sick people (Soucheray, 2020, April 10).

Even if you are not familiar with the 1918 pandemic, you may be aware of the 2009 H1N1 pandemic—the first global influenza pandemic in more than forty years. It was caused by the emergence of a novel* H1N1 influenza strain that reminded us just how serious an influenza pandemic can be. By the time the WHO declared the pandemic officially over in August 2010, the CDC estimated that 43 to 89 million people in the U.S. had become infected. It is estimated that worldwide, between 150,000 and 575,000 people died from 2009 H1N1 virus infection during the first year the virus circulated (CDC, 2021, September 8).

*Novel: New (novel) influenza viruses are different from those currently circulating. This can include highly pathogenic influenza A viruses (H1N1, H5N1, H7N3, and H7N9).

Influenza experts believe that another influenza pandemic will occur—likely caused by an influenza subtype to which there is little or no pre-existing immunity in the human population. Even though the H1N1 pandemic of 2009 is officially over, the H1N1 virus continues to circulate as a seasonal virus and is expected to do so for several years. Fortunately, most (although not all) countries have developed influenza vaccines that protect against the H1N1 virus.