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 a number of 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:
- Cough
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
- Headaches
- 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
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.
Antigenic shift
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 needs to be updated seasonally 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, they begin formulating the latest vaccine. Sometimes, the strains chosen remain dominant that season, other times, the predictions are off, and the flu vaccine proves less effective that year.
This year’s flu vaccine was formulated to cover two different influenza A and two influenza B types, including the H3N2 strain of influenza A. Some very early data (as of Jan. 2021) suggests that the 2021-2022 flu vaccine’s effectiveness against the current predominant H3N2 viruses may be reduced compared to past vaccines due to antigenic drift. However, the CDC and other experts continue to recommend vaccination this year. When compared with unvaccinated people, many vaccinated people this season had antibodies that neutralized the emerging H3N2 virus, meaning the vaccine does provide a level of protection against those H3N2 viruses.
In summary, while it is too early to calculate a percentage of effectiveness this year’s flu vaccine provides, the 20201-22 influenza vaccine does provide some protection against the season’s dominant flu strains, which can prevent severe illness and hospitalization.
What to Expect for the 2021-22 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 2021-22 flu season:
- 2021-2022 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.
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 our find a clinical trial page.
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?
Centers for Disease Control and Prevention, CDC Urges Flu Vaccination as Flu Activity Picks Up
