What is Influenza (Flu)?
Influenza, more commonly known as the flu, is caused by influenza viruses. “The flu” and “influenza” are generally used as umbrella terms for four subtypes of the influenza virus: A, B, C, and D. Influenza A and B are the most common in humans, with Influenza A generally being the most diverse and prevalent, as it can infect several animal species in addition to humans. Since it can infect numerous hosts, the capability for a mutation to develop and create a new strain can make the flu more or less contagious or dangerous from year to year. Influenza C can infect humans, and occasionally dogs and pigs. Influenza D has not been known to infect humans and mostly affects cattle.
The respiratory illness has a wide range of severity, and symptoms often appear suddenly. Common symptoms include:
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
- Fatigue (tiredness)
- Fever or chills (though not everyone who has the flu gets a fever)
- Vomiting and/or diarrhea (more common in children than adults)
Influenza spreads similarly to other respiratory illnesses like COVID-19 or respiratory syncytial virus (RSV) — through airborne droplets that can be created by coughing, sneezing, and even talking. The virus can also survive for hours when it reaches a hard surface, such as a door handle, countertop, bottle, or smartphone.
Why is the Flu Seasonal?
The flu season for the Northern Hemisphere generally starts in October and ends in March. For the Southern Hemisphere, the season begins in April and ends in September — the fall to spring months, respectively. The reason the flu is seasonal is actually unclear, but this article shares three main theories:
- When it’s cold, people spend more time indoors with windows and doors closed, which allows for the droplets of infected people to more easily spread to uninfected people.
- Shorter days mean that many people aren’t in the sun as much, and may not produce sufficient vitamin D and melanin. This compromises the immune system, and makes fighting off the virus more difficult.
- The influenza virus may survive better in colder, drier climates, and therefore be able to infect more people.
Why Does the Flu Fluctuate in Severity Year to Year?
Since the flu is present year-round, spans the globe, and can spread between species of hosts, the virus has opportunities to mutate and change from year to year. There are two main ways that influenza (subtype A, most often) changes — antigenic drift and antigen shift.
Antigenic drift happens when the proteins on the outside of the virus mutate and change shape slowly over time. This change is not very dramatic and usually allows the body’s immune system to recognize and fight the virus off if you have previously had the flu or the vaccine. However, over time, the drifts accumulate enough to significantly change the exterior of the virus, to the point where the body’s immune system does not recognize it and does not trigger an immune response. Antigenic drifts are a main reason people can catch the flu multiple times.
Occasionally, a new mutation in the Influenza A virus can change the proteins on the outside of the virus significantly enough to be considered an antigenic shift. Antigenic shifts This can happen when an animal flu gains the ability to infect humans, like the Swine Flu pandemic in 2009. Pandemics are rare — only four have occurred in the past 100 years, according to the CDC.
Overall, drifts and occasional shifts create new subtypes of the flu often enough that there is a new subtype of flu circulating nearly every year. This means that prior immunity via exposure or vaccination might not protect you from year to year. These changes mean that for many reasons, some flu seasons can be severe, while others are almost non-existent.
Flu Vaccine Effectiveness
Due to antigenic drift and shift, the flu vaccine often needs to be updated to better protect against the most dominant strains at the time. Each year, researchers analyze the globally circulating strains and try to predict which three or four of those strains will be most prevalent in the upcoming influenza season. From those predictions, typically the most prevalent 3 to 4 strains of the flu are included in the vaccines distributed in autumn. Sometimes, the strains chosen remain dominant; other times, when the prevalent strains are not those predicted, the flu vaccine may be less effective.
The FDA has recommended that this year’s flu vaccines cover two different influenza A and two influenza B types. Early data suggests that most cases of the flu in the United States this year are caused by influenza A strains, specifically those within the H3 group of viruses, meaning the vaccines this year should provide some additional aid in preventing infection.
In summary, the 2022-23 flu vaccine should provide some protection against the season’s dominant flu strains.
What to Expect for the 2022-23 Flu Season
The CDC and NIH are the most credible sources of information about flu spread. Below are some of the most helpful links to reference for the upcoming 2022-23 flu season:
- 2022-2023 Flu Season FAQ
- How to Prevent Flu
- Flu Vaccine Safety
- Weekly U.S. Influenza Surveillance Report
Because of COVID-19 guidelines and mandates put in place by the federal, state and local governments regarding social distancing and masking, the flu season seemed almost non-existent in 2020-21. In 2021, the flu began returning to its normal prevalence after the COVID-19 guidelines and mandates were ended.
How Do I Avoid Catching the Flu?
There are a number of ways to protect yourself against influenza each year, and to prevent it from spreading to others. The first is to get vaccinated.
Vaccination from the flu tends to be between 40 to 60% effective year to year at preventing illness. In those who get vaccinated and still catch the flu, the vaccine has been proven to reduce severity of illness and reduce the risk of flu-associated hospitalization. Getting vaccinated can help protect those who cannot due to certain medical conditions (for example, infants and those with severe allergic reactions).
In addition to the flu vaccine, employing practices similar to the COVID-19 protocols of avoiding close contact with people who are sick, maintaining six feet of distance between you and others in public, washing your hands frequently, covering coughs and sneezes, and wearing masks while socializing can dramatically reduce the risk of catching and spreading the flu.
Flu Vaccine Trials at Velocity
Velocity is enrolling for numerous vaccine studies, including influenza and RSV studies. Participants in these studies may receive investigational vaccines and, in some cases, an FDA-approved vaccine at no cost. Eligible participants may also receive compensation for study-related time.
Velocity is always enrolling patients into new clinical trials. To learn more about current and upcoming clinical trials at Velocity, visit velocitypatients.com.
References and Continued Reading
Centers for Disease Control and Prevention, Key Facts about Seasonal Influenza.
Centers for Disease Control and Prevention, Vaccine Effectiveness: How Well Do Flu Vaccines Work?
Harvard University, Science in the News, The Reason for the Season: why flu strikes in winter.
Healthline, How Are Influenza A and B Different?