How Effective Is the Flu Vaccine? See Early Estimates for 2023-2024

Apr 1, 2024

How effective is the flu vaccine this year?

We know that flu vaccines are the best way to protect people from the flu. However, the level of protection that flu vaccines provide—known as flu vaccine effectiveness—changes from year to year. (These changes may be affected by certain things, such as a vaccinated person’s age or health and how well vaccines “match” circulating flu viruses.)

Now that we are a few months into the 2023-2024 flu season, we have a sense of how well vaccines have worked over this period. When we talk about how well vaccines work, it’s important to remember that flu vaccines work against the flu in two ways: First, they help protect people from getting the flu, and second—in instances where vaccinated people still develop flu symptoms—they can decrease the severity of illness and reduce the chances that people will have to see a doctor, get hospitalized or even die due to the flu.

So what’s the bottom line? Flu vaccines this season have worked just as well as—if not better than—vaccines in past flu seasons, providing protection against flu and flu-related complications to people of all ages.

In the 2023-2024 flu season so far, getting a flu vaccine made people less likely to visit a doctor or require hospitalization if they did catch the flu, according to early estimates published by the Centers for Disease Control and Prevention (CDC) at the end of February.

These estimates were found to be true across all age groups—from children aged 6 months to 17 years to adults of all ages. Also, flu vaccines were shown to be effective against the Influenza A and Influenza B strains that have been circulating this season.

Estimates on this season’s flu vaccine effectiveness were presented at a meeting of the CDC’s Advisory Committee on Immunization Practices (ACIP) in late February.

“We’re right in the range that we typically see when the vaccine is a good match with the viruses that are circulating,” said Sascha Ellington of the CDC’s Influenza Prevention and Control, Influenza Division, according to CBS News. “Good [vaccine effectiveness], and it’s working consistent with past years.”

The CDC still recommends getting a flu vaccine as long as the flu is circulating. As of the week ending March 16, the CDC reported in one of its weekly flu updates that flu cases have declined since late December. However, while the number of flu-related visits to the doctor have declined, flu cases remain above baseline levels.

Flu vaccine effectiveness in children this season

Children are at high risk for serious flu complications due to their age, as well as any medical condition they may have. As of the week ending March 16, the CDC reports that 121 children have died due to the flu this season. Looking further into these early estimates, we see that children who were vaccinated for the flu this flu season were:

About 59% to 67% less likely to visit a healthcare provider due to the flu
About 52% to 61% less likely to be hospitalized for the flu

Some data about flu vaccine effectiveness in children who caught a strain of Influenza A and Influenza B is available. Vaccinated children who caught an Influenza A virus were:

46% to 59% less likely to see a healthcare professional for their flu symptoms
46% to 56% less likely to require hospitalization due to flu

Vaccinated children who developed the flu from the Influenza B/Victoria virus were 64% to 89% less likely to see a healthcare provider due to the flu.

Flu vaccine effectiveness in older adults this season

People who are aged 65 years and older tend to be at the highest risk for flu complications. This season, estimates suggest that those older adults who received a flu vaccine were:

About 41% to 51% less likely to visit a healthcare provider due to the flu
About 42% less likely to be hospitalized with the flu

Similar to children, vaccination also helped older adults avoid outpatient visits or hospitalization due to Influenza A. Vaccinated older adults were 40% to 52% less likely to see a healthcare provider and 42% to 47% less likely to be hospitalized due to infection with an Influenza A virus. They were also 69% less likely to see a healthcare provider due to Influenza B/Victoria.

Flu vaccine effectiveness in all adults this season

Vaccination also provided protection against flu and flu complications for adults aged 18 years and older. This group of adults who got a flu shot this season were:

About 33% to 49% less likely to see a healthcare provider because of flu
About 41% to 44% less likely to be hospitalized for the flu

Vaccinated adults who were infected with an Influenza A virus were 27% to 46% less likely to have an outpatient healthcare visit and 40% to 42% less likely to require hospitalization for their flu symptoms. They were also 78% less likely to require a healthcare visit and 60% less likely to need hospitalization due to Influenza B/Victoria.

Getting a flu shot this season has reduced the chances that people will visit a healthcare provider—or require hospitalization—due to the flu

What affects flu vaccine effectiveness?

As mentioned earlier, flu vaccine effectiveness depends on several things, such as certain characteristics of the person who gets vaccinated. For example, some people are at higher risk for flu-related complications based on their age, such as young children and adults aged 65 and older. Young children may also have chronic health conditions, and older adults often have a weaker immune response to vaccination.

Yet how effective the flu vaccine is in preventing flu and flu complications each year can also depend on what flu strains are circulating. The “match” that flu vaccines have with circulating strains will also affect flu vaccine effectiveness that season. However, even when flu vaccines aren’t well matched to strains in circulation, they may still provide protection against other flu strains.

How the CDC tracks flu vaccine effectiveness

Flu vaccine effectiveness is monitored each year through CDC vaccine effectiveness studies, which are conducted through several healthcare networks. The data gathered through these studies to create the CDC’s estimates was collected from people who made outpatient visits or had to be hospitalized due to the flu.

Four healthcare networks were involved data collection in 22 states for the 2023–2024 flu season, including:

Investigating Respiratory Viruses in the Acutely Ill (IVY)
New Vaccine Surveillance Network (NVSN)
U.S. Flu Vaccine Effectiveness (VE) Network
VISION Vaccine Effectiveness (VE) Network

Of these, the U.S. Flu VE Network is the oldest, having been set up in 2003–2004. That flu season marked the first time that the CDC began collaborating with university and hospital researchers to conduct studies on how effective flu vaccines are within a flu season.

The difference between flu vaccine effectiveness and flu vaccine efficacy

The studies conducted through these CDC networks on flu vaccine effectiveness are observational studies, which measure how well vaccines work in real-world settings. These studies are different from the randomized controlled trials that allow investigators to research flu vaccine efficacy. According to the CDC, vaccine efficacy studies are less likely to have results affected by biases than vaccine effectiveness studies. This is why flu vaccine licensing depends on results from the randomized controlled trials used to measure vaccine efficacy.

Review 2023–2024 flu vaccine effectiveness data collected from the CDC’s networks