Why Are Some Flu Seasons Worse Than Others?

Ridofranz/istock 

It seems like every flu season is different. Some years the flu is really severe and makes a lot of people sick and other years it's not so bad. Influenza (the flu virus) has been making people sick for hundreds of years, so why is it so unpredictable?

Understanding Flu Strains

The flu can be caused by any of hundreds of strains of the influenza virus.

Typically, one or two strains will dominate during any given flu season. Unfortunately, the virus mutates and changes fairly frequently, so we never know which strain will dominate. 

Although there are hundreds of strains of influenza and they are grouped into types and subtypes. Influenza A, for instance, is the most common type of flu that causes illness in humans. Influenza A is further subtyped into H#N# groups. For example, the most recent flu pandemic occurred in 2009-10. It started because a strain of H1N1 that had previously infected primarily pigs mutated and began infecting humans. It was a type of flu that most humans had little to no immunity against, so it sickened a large portion of the world's population. Fortunately, it was not as deadly as some previous flu pandemics, such as the one in 1918 that killed as much as 5 percent of the world's population. 

Even when we don't have a pandemic strain of flu, there are differences in the severity of types of seasonal influenza.

Influenza A strains are usually more serious than influenza B. Influenza C can also cause illness in humans, but the symptoms are typically mild like those of a cold, so this type of flu isn't identified very often because people don't seek medical care when they have it. Further, certain subtypes of influenza A have been known to cause more severe flu seasons than others.

In years when an H3N2 virus is the dominant strain, we often see higher hospitalization rates and more deaths than in years when another type of flu is making people sick. 

Flu Vaccine Efficacy

The US Centers for Disease Control and Prevention (CDC) reports that flu vaccine efficacy varies between 40 percent and 60 percent when the vaccine is a good match for the circulating strain of the virus. This number is often lower during years when the vaccine is not a good match. Although that is significantly lower than the rate of efficacy for most other vaccines, it's still better than not getting vaccinated at all. Studies have shown that people who are vaccinated (especially older adults and young children) are less likely to be seriously ill, hospitalized, or die when they get the flu than those who have not been vaccinated.

It can be frustrating to get sick with the flu even after you got the flu shot, but that doesn't mean it isn't worth getting. You would likely be more ill if you hadn't been vaccinated. There are many people that 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.

What You Can Do

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

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 can also be spread through the air and by touching objects where the influenza virus has landed. 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 on your hands when used correctly. 
  • Don't Share Utensils - This is a lesson most people learn during childhood, but sometimes we fall back into old habits with people we know well. Unfortunately, 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, spoons, and other products that may go in your mouth or nose to yourself. 

Lessons Learned

During years when the flu vaccine is not well matched to the strain of influenza that is circulating in the community, it can be frustrating for everyone. Those who were vaccinated may still get sick, those who weren't will claim getting the vaccine is useless because "it doesn't work anyway," and public health officials are blamed for not knowing what was coming. 

Of course, no one can see the future, and unfortunately the flu vaccines we currently have only target specific strains. Until universal flu vaccines are available, we will have to do the best we can with what we have. Education is essential so people understand how deadly the flu can be and how flu vaccines save lives even when they aren't a good match to the circulating virus. Even during years when the vaccine is not a good match, a majority of deaths and hospitalizations are among people that are unvaccinated. 

A Word From Verywell

Science has not advanced to the point where we can identify which strains of influenza will cause illness before it happens. Influenza viruses mutate and change frequently, making it difficult to keep up. The flu vaccines we currently have are not perfect and are only specific to the strains of the virus that are included in them, which may or may not be the strain(s) that is making people sick during a given year. Still, it is the best protection we have and repeated studies have shown that getting the vaccine keeps people out of the hospital and saves lives even when it isn't a good match. 

More work and research is needed—and is being done—to develop better flu vaccines that hopefully will not be needed every single year. Until then, education is key. Be sure you understand how serious the flu can be, how it is spread, and how you can protect yourself and your family from getting sick. 

Sources:

Arriola C, Garg S, Anderson EJ, et al. Influenza Vaccination Modifies Disease Severity Among Community-dwelling Adults Hospitalized With Influenza. Clin Infect Dis. 2017;65(8):1289-1297. doi:10.1093/cid/cix468

CDC. Key Facts About Seasonal Flu Vaccine. Centers for Disease Control and Prevention. https://www.cdc.gov/flu/protect/keyfacts.htm. Published October 30, 2017.

Flannery B, Reynolds SB, Blanton L, et al. Influenza Vaccine Effectiveness Against Pediatric Deaths: 2010–2014. Pediatrics. April 2017:e20164244. doi:10.1542/peds.2016-4244

Grohskopf LA. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2017–18 Influenza Season. MMWR Recomm Rep. 2017;66. doi:10.15585/mmwr.rr6602a1

Vaccine Effectiveness - How Well Does the Flu Vaccine Work? | Seasonal Influenza (Flu) | CDC. https://www.cdc.gov/flu/about/qa/vaccineeffect.htm.