The effectiveness of the flu vaccine varies each year, depending on how well the strains selected for inclusion in the vaccine match those that ultimately circulate, but on average is around 40% in preventing an outpatient medical visit due to lab-confirmed flu. Flu vaccines also tend to not work as well in older people, who are more vulnerable to severe influenza. For these reasons, Georgetown’s Goodman said there are “lots of efforts to try to make improved influenza vaccines.”
Still, the shots, which only about half of Americans get every year, prevent millions of illnesses and thousands of deaths each season — and could have an even bigger impact if more people got them. Flu vaccines are most important for those at high risk, but are a good idea for everyone.
It’s not entirely clear what the 2022-2023 flu season will look like, but CDC surveillance data as of the end of last week suggests the season is getting an early start and could be worse than normal. In some parts of the country, particularly in the South, flu activity is already high. So far, it appears that this year’s vaccine is a good match for the circulating influenza strains, but fewer people have been vaccinated than in the past couple of years.
This year, for the first time, the CDC is preferentially recommending that people 65 years of age and older get a high dose or adjuvanted flu vaccine, since there is evidence that those shots may work better than standard flu shots in that group.
How the CDC Estimates Flu Deaths
As part of Malone’s effort to undermine confidence in flu shots, he incorrectly claimed that the CDC classifies all pneumonia deaths as flu deaths “in order to increase the perceived risk of influenza virus” — and said the agency has “grossly overstated the threat of influenza.”
It’s true that one way the CDC keeps track of the flu burden is by looking at a composite measure that includes illnesses and deaths from influenza and pneumonia. But the agency doesn’t claim that all of those deaths are from influenza — and it’s not collecting the data to artificially increase flu deaths. Rather, because flu deaths are well known to be underreported, the agency uses established statistical methods to estimate how many people have died of influenza each year.
“The CDC has historically grouped pneumonia and influenza mortality together (in some — not all — metrics it uses) because fatal influenza causes pneumonia and it is difficult to tease the two apart especially when many influenza cases are not specifically diagnosed,” Adalja said. “It is also the case that during influenza seasons, there is a clear mortality increase that is strongly linked to influenza activity.”
In a statement, the agency told us it does not “count all pneumonia deaths as influenza deaths in an effort to inflate influenza burden numbers.”
“Because pneumonia is a common complication of influenza, CDC does combine counts of pneumonia & influenza (P&I) deaths to monitor weekly trends in mortality during the fall and winter influenza season. These trends are presented in the weekly surveillance report FluView,” the agency continued. “However, that is separate from CDC’s estimates of annual influenza deaths, which are based on 5 decades of statistical modeling that CDC has undertaken to convey an accurate picture of the burden of flu influenza.”
“There were earlier studies, 60+ years ago, where P&I deaths above a baseline were assumed to be influenza if they occurred during the typical season. Improvements to this methodology have been developed to better attribute excess deaths to influenza using viral surveillance data,” the CDC added. “However, that methodology has not been used by CDC to estimate the burden of influenza deaths in the U.S. population in more than a decade.”
We reached out to Malone to ask for support for his claims, but did not receive a reply.
His claim about flu is similar to those made about COVID-19 when the agency began tracking COVID-19 deaths along with those from pneumonia and influenza as a mortality surveillance measure. As we have written, the metric is used for monitoring purposes, and the agency was not counting all such deaths as COVID-19 deaths.
A CDC website explains that the agency estimates the number of flu deaths using a model that applies a ratio of deaths-to-hospitalizations. First, the agency looks at in-hospital laboratory-confirmed deaths recorded in FluSurv-NET, which covers a network of hospitals in 14 states. That number is adjusted for under-detection of influenza by factoring in how often people with respiratory illnesses were tested for influenza, and how sensitive those tests are.
Next, the agency looks at death certificates that have pneumonia or influenza causes, or others, since not all influenza deaths will have occurred in a hospital or have had influenza listed as a cause of death. Rather than including all pneumonia and influenza deaths, as Malone alleged, the agency says that it uses information on the causes of death from FluSurv-NET to determine how many of the non-hospitalized pneumonia and influenza or other deaths to include. Once the agency scientists estimate the proportion of influenza-associated deaths that happened outside a hospital, they estimate the deaths-to-hospitalization ratio.
The method isn’t perfect — and the agency doesn’t claim to provide an exact number, only an estimate — but the approach is necessary because otherwise, flu-related deaths would be vastly underreported.
“CDC feels it is important to convey the full burden of seasonal flu and thus uses well-established scientific methods that have been reviewed by scientists outside of CDC to make comprehensive estimates of the total numbers of annual flu illnesses, flu-associated hospitalizations, and flu-associated deaths that occur each year,” an agency webpage reads.
The CDC notes that most people are not tested for influenza or may seek medical care at a point when tests can no longer easily detect the virus. The tests are also not particularly sensitive to begin with. States are also only required to report flu deaths in children, not adults. Death certificates often fail to mention flu, not only because of the lack of testing, but because many people die from complications weeks after an infection, such as after developing a secondary bacterial infection or from aggravation of a chronic illness.
“Only counting deaths where influenza was recorded on a death certificate would be a gross underestimation of influenza’s true impact,” a CDC page reads.
Editor’s note: SciCheck’s COVID-19/Vaccination Project is made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over FactCheck.org’s editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation. The goal of the project is to increase exposure to accurate information about COVID-19 and vaccines, while decreasing the impact of misinformation.
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