Surging or Shackled? Flu Season Could Go Either Way

Infection Control TodayInfection Control Today, October 2021 (Vol. 25 No. 7)
Volume 25
Issue 8

The season of respiratory tract infections is upon us. Influenza, rhinovirus, respiratory syncytial virus, and pertussis—as well as COVID-19—once again are attacking many individuals across the US and worldwide.

The vaccine protects those who don’t have major underlying health conditions such as diabetes, cancer, and autoimmune diseases. Nonetheless, the season of respiratory tract infections is upon us. Influenza, rhinovirus, respiratory syncytial virus, and pertussis—as well as COVID-19—have begun to attack many individuals across not only the United States but also the entire world. We saw this coming. With the decrease in mask mandates this year, we should not be surprised by an increase in respiratory tract infections, including COVID-19 and influenza, both of which can cause an increase in hospitalization and death.1

There are environmental factors at play, from the red tide in Florida to wildfires and poor air quality across the state of California. Many areas throughout the country are dealing with higher-than-normal temperatures. In the coming influenza season, the questions to be answered are whether people will get vaccinated against influenza and whether the heated debates over COVID-19 vaccinations (to mandate or not mandate) will drive more individuals to get their influenza vaccine this year.

When historians talk about the history of influenza, they inevitably bring up the Spanish flu of 1918, which infected approximately 500 million people around the world and killed an estimated 50 million.2 But influenza actually goes back to the ancient Greeks, to the time of Hippocrates in the fifth century BC. Influenza has been around, so to speak, for 2500 years, at least.3

Although the illness can strike at any time of year, the typical flu season usually starts in September or October with a peak between December and February. Once an individual has been infected with the virus, viral shedding begins within 24 to 48 hours, and it usually takes about 24 hours before symptoms begin. These can include fever or feeling feverish; a stuffy or runny nose; cough; muscle and body aches; headache; fatigue; and vomiting and diarrhea, which are more typical in children than in adults. Influenza can range from asymptomatic to severe illness. Symptoms of COVID-19 can be similar but take longer after exposure to manifest, usually between 2 and 14 days but typically by day 5.

Each year, state and local health department laboratories submit data to the Centers for Disease Control and Prevention (CDC) about the present year’s influenza cases. The CDC performs genetic and antigenic characterization based on that information. They use these data to compare how similar they are to the current virus in order to target strains in the current vaccine.

Low Activity

There was unusually low influenza activity throughout the 2020-2021 flu season, not only in the United States but also globally. US reports indicated that 1675 (0.2%) of the 818,939 respiratory specimens submitted for testing were positive for an influenza virus. This low level of activity decreased hospitalizations and death compared with previous flu seasons. The number of influenza-associated hospitalizations was the lowest recorded since these data were first collected in 2005. The CDC received 1 report of a pediatric flu death in the 2020-2021 season. The CDC states that “since flu deaths in children became nationally notifiable in 2004, reported flu deaths in children had previously ranged from a low of 37 (during 2011-2012) to a high of 199 (during 2019-2020).” Influenza vaccination also contributed to keeping cases lower, as a record 193.8 million doses of vaccine were administered during 2020-2021.4

Influenza viruses constantly change, so vaccines must change annually to keep people safe and protect them from life-threatening symptoms. During the past flu season (2020-2021), the dominant viruses reported were influenza A (61.4%) and influenza B (38.6%). Most cases of influenza A were of the H3N2 strain, which comprised 52.5% of cases. Sixty percent of cases of the influenza B virus were of the Victoria lineage— B/Victoria/2/87—1 of the 2 major hemagglutinin (HA) lineages of influenza B virus. The other is the Yamagata lineage (B/Yamagata/16/88).3

There were also 5 cases of influenza reported that spreads in pigs and not people. This is known as a variant influenza virus. Those infected reported that they had had direct exposure to pigs or lived on a property where pigs were housed. There was no person-to-person spread of this variant influenza. These types of influenza variants are rare. Since 2005, a total of 489 variant influenza virus infections have been identified in the United States.

After spending the last 2 years hearing so much about the COVID-19 variants, people may ask more questions about influenza variants this year. So let’s look at how and why influenza viruses are constantly changing, which leads to the need for annual vaccinations. The more information we have in our knowledge toolkits, the better prepared we will be when unusual and unexpected questions arise. Most people know the signs and symptoms of influenza, but how many know why this constant change in virus happens?

Antigenic Drift

Influenza viruses change in 2 different ways. The first is antigenic drift. This is when small changes or mutations occur in the virus’ genes that lead to a change in the surface proteins of the virus, HA and neuraminidase (NA). These virus proteins are antigens.5 Antigens are recognized by our immune system and trigger an immune response, which produces antibodies that block an infection. As the virus replicates, antigenic drift occurs. Influenza vaccines are designed to target an influenza virus’ HA surface proteins or antigens. Small changes to the antigenic drift can accumulate over time and result in viruses that are different. When the antigenic drift happens, the body’s immune system may not recognize and prevent illness caused by the new influenza virus. This makes individuals more susceptible to influenza infection again. The virus has changed enough that your existing antibodies won’t attack the newer influenza viruses. This is the primary reason the vaccine must be updated each year.

Antigenic Shift

The second type of change is known as the antigenic shift. The is an abrupt major change in an influenza A virus resulting in a new HA or new HA and NA proteins in the viruses. A shift can result in a new influenza A subtype in humans, such as when an influenza virus from an animal population can infect humans. This occurred in 2009 when the H1N1 virus with genes from North American swine, Eurasian swine, humans, and birds infected people and quickly spread, causing a pandemic.

When a shift happens, most people have little to no immunity against the virus. Antigenic shifts occur less frequently than antigenic drifts.

Four pandemics have been caused by influenza in the past 100 years. A pandemic happens when a nonhuman influenza virus changes, making it more easily transmitted from person to person. Past pandemics occurred in:

  • 1918, caused by the H1N1 virus;
  • 1957-1958, caused by the H2N2 virus;
  • 1968, caused by the H3N2 virus; and
  • 2009, caused by the H1N1pdm09 virus.

By now, you may be thinking that this all sounds like COVID-19, and you would be correct. The more COVID-19 mutates and develops new variants, the less likely it is that the current vaccines will remain as effective. Influenza has already proved in the previous pandemics that viruses are very effective at adapting, and millions of human incubators are all they need. For 2021-2022, the FDA’s Vaccines and Related Biological Products Advisory Committeemade recommendations for egg-based, cell-based, and recombinant influenza vaccines, as listed below6::

US quadrivalent formulations of egg-based influenza vaccines should contain the following:

  • A/Victoria/2570/2019 (H1N1) pdm09-like virus
  • A/Cambodia/e0826360/2020 (H3N2)-like virus
  • B/Washington/02/2019-like virus (B/Victoria lineage)
  • B/Phuket/3073/2013-like virus (B/Yamagata lineage)

US quadrivalent cell- or recombinant-based vaccines should contain the following:

  1. A/Wisconsin/588/2019 (H1N1) pdm09-like virus
  2. A/Cambodia/e0826360/2020 (H3N2)-like virus
  3. B/Washington/02/2019-like virus (B/Victoria lineage)
  4. B/Phuket/3073/2013-like virus (B/Yamagata lineage)

Now, as data are submitted, the CDC will determine how close of a match these vaccines were and how much coverage individuals will need to protect them from influenza. This determines how bad of a flu season the CDC expects to see. The agency then shares its information with state and local officials so proper mitigation processes can be recommended to the general population.7

LINDA SPAULDING, RN-BC, CIC, BC, CHEC, CHOP, is an infection prevention consultant and founder of InCo and Associates International, Inc.

CONNIE HENRY, BSN, RNC, has been a nurse for 35 years, working in the areas of obstetrics, neonatal care, and infection prevention and control.


  1. National Center for Immunization and Respiratory Diseases (NCIRD). CDC. Updated July 6, 2020. Accessed August 23,2021.
  2. 1918 pandemic (H1N1 virus). CDC. Updated March 20, 2019. Accessed August 24, 2021.
  3. Past pandemics. CDC August 10, 2018. Accessed August 20, 2021,
  4. 2020-2021 flu season summary. CDC. Updated July 22, 2021. Accessed August 19, 2021.
  5. How the flu virus can change: “drift” and “shift.” CDC. Updated October 15, 2019. Accessed August 20, 2021.
  6. Influenza vaccine for the 2021-2022 season. United States Food and Drug Administration. Updated September 2, 2021. Accessed August 19, 2021,
  7. Frequently asked influenza (flu) questions: 2021-2022 season. CDC. Updated September 1, 2021. Accessed August 23, 2021.

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