Why Are Some Flu Seasons Worse Than Others?

Why Some Flu Season Are Worse Than Others

Verywell / Brianna Gilmartin

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Not only are there different types and strains of flu, but every flu season varies, with some years being worse than others in terms of number of people infected and the severity of their illness. How mild or severe a flu season is depends on a few different factors, including vaccination rates and vaccine efficacy.

Given that different strains of influenza mutate, scientists are constantly chasing a moving target when it comes to helping make the next flu season better than the last.

Flu Rates: 2010 to 2020

First, it's important to note that the flu is not getting worse every year. Rates of infection and associated deaths rise and fall depending on:

  • The flu strain that's going around
  • How well the vaccine matches the strain
  • How many people get vaccinated

The Centers for Disease Control and Prevention (CDC) reports these numbers for the decade between 2010 and 2020:

Flu Infection Rates and Associated Deaths
Flu Season Infections Deaths
2010-11 21 million 37,000
2011-12 9.3 million 12,000
2012-13 34 million 43,000
2013-14 30 million 38,000
2014-15 30 million 51,000
2015-16 24 million 23,000
2016-17 29 million 38,000
2017-18 45 million* 61,000*
2018-19 35.5 million* 34,000*
2019-20 22-31 million* 12,000-30,000*
*Estimates. Data not finalized for all years.

As you can see, there were relatively low rates during the 2011 to 2012 flu season, somewhat consistent rates for a few years, and then a big spike in 2017 to 2018.

While the number of infections is obviously important, the severity of those cases must also be considered.

For example, reference the rates for the 2013 to 2014 and 2014 to 2015 seasons above. Both saw 30 million people come down with the flu, but in 2013 to 2014, 38,000 people died from it. The following year, the flu killed 51,000.

No estimates are available for the 2020-2021 flu season, because there was "minimal flu activity" due to COVID-19 precautions.

Flu Strains

The human influenza virus comes in three forms:

  • Influenza A
  • Influenza B
  • Influenza C

Every year, you hear about flu viruses with names like H1N1 or H3N2. Those are specific strains of the flu, which are genetically different from other strains.

H1N1, H3N2, and other similarly named influenza viruses are all variations of the influenza A virus, which is the most contagious and most severe of the three influenza viruses that infect humans.

Influenza B is the next most contagious and severe. Influenza C is very mild and not terribly contagious.

Note: Influenza D is the fourth type of flu. While isolated from swine and cattle and not yet passed to humans, researchers do believe this may be possible.

Flu Vaccine Efficacy

Every year, scientists predict what strains will be going around the next flu season, and they create a vaccine based on their predictions. The vaccine always contains two strains of influenza A and one or two strains of influenza B.

The influenza A viruses mutate quickly and unpredictably, so making the prediction isn't easy. On top of that, they need to make that prediction in February in order for the vaccine to be manufactured and distributed before the next flu season begins in October.

The CDC says the flu vaccine is between 40% and 60% effective in years when the vaccine is a good match for the circulating strain of the virus. However, the 2019 to 2020 vaccine fell short of the 40% mark for the H3N2 virus, which was a major strain that season. That's because H3N2 mutates faster than H1N1 or influenza B, making it hard to predict.

No estimate was available for the 2020-2021 season. In 2021-2022, vaccine effectiveness was estimated at 35% because the circulating H3N2 strains were again somewhat different from the vaccine strains.

Although flu vaccines are significantly less effective than most other vaccines, getting one is still better than not getting one at all. Studies have shown that people who are vaccinated (especially older adults and young children) are less likely to be seriously ill, be hospitalized, or die when they get the flu than those who have not been vaccinated. Even during years when the vaccine is not a good match for the circulating flu strains, a majority of deaths and hospitalizations are among people who were unvaccinated.

Many people are at high risk for flu complications and could easily lose their lives to the flu if they get sick. If you don't get vaccinated to protect yourself, do it for someone that you care about who could be seriously affected by the flu.

If you get sick with a flu-like illness after you got the flu shot, don't assume the vaccine wasn't effective. You may have had:

  • A milder case because you got vaccinated
  • A strain that wasn't in the vaccine, such as influenza C
  • An illness with similar symptoms caused by a virus unrelated to influenza

Fighting Flu

The most important thing you should do to protect yourself each year is to get the flu vaccine. The more people that are vaccinated, the safer everyone is.

Other important steps to take:

  • Wash your hands: Proper hand hygiene is the best way to prevent the spread of all infections, not just the flu.
  • Avoid touching your face: The flu is spread primarily through droplets when people cough and sneeze, but it can also be spread through the air and by touching objects the influenza virus has landed on. The more often you touch your face, the more opportunities germs have to enter your body through your nose, mouth, and eyes.
  • Get plenty of sleep: Rest gives your body time to heal and recuperate. Getting enough sleep every night is essential to making sure your body is able to fight off any illness that you may encounter.
  • Use hand sanitizer: When soap and water aren't available, hand sanitizer is a great way to prevent the spread of germs. As long as your hands aren't visibly soiled, it can effectively kill a majority of germs when used correctly.
  • Don't share utensils: This can lead to the spread of germs and illness, since some viruses, like the flu, are spread before we even know we are sick. Keep your drinks, forks, knives, and spoons to yourself.

Cold/Flu Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Man

A Word From Verywell

Someday, a flu vaccine that's more like other vaccines—one shot that protects you from most or all strains, and for many years—may be available. For now, it is important to get a flu shot every October in order to protect your health. The CDC publishes a weekly report on the state of the flu in the United States if you'd like to stay up-to-date on the current flu season.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Burden of influenza.

  2. Centers for Disease Control and Prevention. Vaccine Effectiveness: How Well Do the Flu Vaccines Work?

  3. Centers for Disease Control and Prevention. Interim US Flu Vaccine Effectiveness (VE) Data for 2021-2022.

  4. Flannery B, Reynolds SB, Blanton L, et al. Influenza vaccine effectiveness against pediatric deaths: 2010–2014. Pediatrics. 139(5):e20164244. doi:10.1542/peds.2016-4244

Additional Reading

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.