UAB Medicine News


Flu Strains Explained, and How the Vaccine Works

As we approach flu season, UAB Medicine already is getting questions about the different strains of the flu virus and the vaccine itself. With expert input from Rachael Lee, MD, assistant professor in the UAB Division of Infectious Diseases, we explain the differences in flu strains, including how many strains exist and what the flu shot will cover this year. We also answer some common questions, such as whether the flu shot for one strain reduces the risk of contracting another strain and whether flu vaccines are reissued after the virus mutates during the course of the season.

Understanding Flu Types

According to the Centers for Disease Control and Prevention (CDC), there are four types of flu viruses: influenza A, B, C, and D. The seasonal flu viruses that humans face every winter season in the United States are caused by human influenza A. Influenza A viruses are categorized as either the hemagglutinin subtype or the neuraminidase subtype based on the proteins involved, and there are 18 distinct subtypes of hemagglutinin and 11 distinct subtypes of neuraminidase. Influenza A is the primary cause of flu epidemics, and they constantly change and are difficult to predict.

Meanwhile, influenza B viruses also affect humans and can be categorized by different strains and lineages. Influenza C viruses can cause mild respiratory illnesses but not epidemics, and modern flu vaccines do not protect against them. Finally, influenza D viruses primarily affect cattle and are not a problem for humans.

This Year’s Flu Shot

Dr. Lee says this year’s flu vaccine covers four strains of the flu that are recommended by the World Health Organization. The strains recommended for vaccination for the 2020-2021 flu season in the northern hemisphere are:

  • A/Hawaii/70/2019 (H1N1)pdm09-like virus
  • A/Hong Kong/45/2019 (H3N2)-like virus
  • B/Washington/02/2019 (B/Victoria lineage)-like virus
  • B/Phuket/3073/2013-like (Yamagata lineage) virus

However, flu viruses tend to mutate during flu season, which may alter the effectiveness of the current flu vaccine. Unfortunately, manufacturers are not able to reissue flu vaccines when this happens because of the way flu vaccines are created and produced for human use.

“It takes months to manufacture the vaccine, so manufacturers cannot change what has already been decided in February before the start of the flu season annually,” Dr. Lee says. “The flu strains were decided in February 2018 for the upcoming flu season.”

Flu Shot Effectiveness

Every year, the CDC conducts studies about how effective the flu shot is at protecting people against the virus, but recent statistics show that the vaccine is 40-60% effective in reducing the risk of flu illness. Thus far, the vaccine has been most effective in protecting against the influenza A (H1N1) and influenza B viruses, so there is still room for improvement in preventing influenza A (H3N2) viruses.

The effectiveness of the flu vaccine varies from year to year and also may depend upon the age and health of the person receiving the vaccine. It also depends upon the similarity between the actual flu virus affecting a community and the specific flu viruses that the current year’s vaccine was manufactured to protect against. Unfortunately for the 2018-2019 season, the overall effectiveness was low at 29%, which is why many of the strains were changed in the upcoming influenza vaccine for the 2020-2021 flu season[LR(1] .

Does the flu shot for one strain help minimize the effects from a flu infection caused by another strain? “This really depends on how similar the two strains are to each other, but usually not,” Dr. Lee says. “That's why the flu vaccine contains two strains of influenza A and two strains of influenza B.”

Benefits of the Vaccine

Regardless of which flu strains cause an epidemic, there are undeniable benefits to receiving the flu vaccine, especially for those with chronic illness, pregnant women, infants, and the elderly. Even if you are infected by a strain of the flu that isn’t covered by this year’s vaccine, studies show that your symptoms may be milder and that you may recover faster. In short, it’s in your best interest to take every recommended precaution to prevent or minimize a flu infection.