Actual discussion:

Parent “I want vaccine for my child”

Doc “There isn’t one, but we have

Parent “We don’t believe in that”

So much of what we are for at Science-Based Medicine is reflected in what we oppose: all the pseudo-medical interventions and SCAMs. It is not always a positive message, forever noting why you should not be participating in a given bit of fantasy-based medicine. We are often the nay-saying curmudgeons of the medical world. Even for medical topics about for which I am in strongly in favor, vaccines, much of my prose is devoted to countering myths and lies about influenza vaccines, from why the Cochrane review is messed up to why health care workers are dumb asses for not being vaccinated.

Well no Debbie Downer or Crotchety Crislip today. Nope. We are going to ride our Rainbow Unicorn to the land of Happiness and Immunity and discuss some of the reasons why you and yours should get the influenza vaccine.

There are many fine points about the flu vaccine that makes conclusions from the literature problematic: what are the circulating strains, what strains are in the vaccine, who is getting vaccinated and the quality of the studies. Influenza is a complex and complicated disease. If you are a preponderance-of-literature kind of person, as I am, you may find that the 23,600 plus articles on influenza vaccination mostly point to widespread moderate benefit of influenza vaccination, although, as always in medicine, for every study you can find an equal and opposite study and the quality of the studies are variable. At the conclusion you will find links to the other influenza related posts I have written.

The flu vaccine decreases your chance of getting the flu. Note decreases chances, not prevents. That remains the worst aspect of the flu vaccine: it is not as effective as other vaccines. But it does prevent the influenza. Even the biased, cherry picking, awful Cochrane review has to admit efficacy:

Parenterally administered influenza vaccines appear significantly better than their comparators and can reduce the risk of developing influenza symptoms by around 4%, if the WHO recommendations are adhered to and the match is right. However, whilst the vaccines do prevent influenza symptoms, this is only one part of the spectrum of “clinical effectiveness” as they reduce the risk of total “clinical” seasonal influenza (i.e. influenza-like illness) symptoms by around 1%.


While the parenteral vaccine efficacy against seasonal (i.e. non- pandemic) influenza is around 75% for the WHO recommended and matched strain, its impact on the global incidence of clinical cases of influenza (i.e. ILI) is limited (around 16% in best case scenario).

despite their disingenuous attempts to sabotage their results with anti-vaccine editorializing.

Sorry. Kinder and gentler. But when I re-read their meta-analysis, my Rainbow Unicorn began to sob.

The CDC estimates vaccine efficacy at around 60%, although it varies from year to year and from population to population.

Vaccination was significantly effective against laboratory confirmed influenza during sporadic activity (odds ratio [OR] 0•69, 95% CI 0•48—0•99) only when the vaccine matched. Additionally, vaccination was significantly effective during regional (match: OR 0•42, 95% CI 0•30—0•60; mismatch: OR 0•57, 95% CI 0•41—0•79) and widespread (match: 0•54, 0•46—0•62; mismatch: OR 0•72, 95% CI 0•60—0•85) outbreaks.

In the elderly the high-dose influenza vaccine drops the risk of influenza although the standard vaccination has efficacy as well. The vaccine has variable efficacy in other populations as well: dialysis patients, diabetics, and immunosuppressed adults with cancer to name a few. I could not find a population where influenza vaccination did not have some benefit, although it does not provide perfect protection.

In a highly-vaccinated, closed population (a Navy ship), there was an outbreak of influenza when almost 100% of the seamen were vaccinated. 24% came down with flu. Horrible efficacy, right? But in nonimmune populations the attack rate can be has high as 80%, so it is better to have a quarter of you population ill than four-fifths.

Even if you get the flu, the data is suggestive that you will have a milder illness. And a milder illness may mean fewer complications, less spread of disease and fewer hospitalizations.

I know there is a strong ‘screw you’ attitude in the US. I got mine, I don’t care about you. It used to be a rising tide lifted all boats, now it sometimes seems that many are not interested participating in activities that, while it may not be of personal benefit, it might benefit those around you. With infectious diseases, having the herd maximally immune is as important as having the individual immune. It protects those who cannot protect themselves.

One of my favorite examples of the population effects of flu vaccination was in Canada where one province, Ontario, had a much higher influenza vaccination rate than other provinces and the results were impressive: a decrease in influenza-associated mortality and health care use.

As they suggest:

The results of this large-scale natural experiment suggest that universal vaccination may be an effective public health measure for reducing the annual burden of influenza.

Even with only modest efficacy, when applied to large populations vaccination leads to significant decreases in morbidity and mortality:

We estimated that during our 6-year study period, the number of influenza illnesses averted by vaccination ranged from a low of approximately 1.1 million (95% confidence interval (CI) 0.6-1.7 million) during the 2006-2007 season to a high of 5 million (CI 2.9-8.6 million) during the 2010-2011 season while the number of averted hospitalizations ranged from a low of 7,700 (CI 3,700-14,100) in 2009-2010 to a high of 40,400 (CI 20,800-73,000) in 2010-2011.

There are those who dismiss the morbidity and mortality of influenza:

As healthy adults have a low risk of complications due to respiratory disease, the use of the vaccine may be only advised as an individual protection measure against symptoms in specific cases.

It is important to remember that with infectious diseases you are vaccinating not only individuals but populations. When you get a vaccine you may also be protecting others from disease, and the effects of herd immunity are evident when you look at the effects of vaccination on the health of populations.

While the Cochrane review mentions that vaccination did not prevent influenza transmission, that was only because they did not include the studies that demonstrate vaccination leads to decreased influenza transmission. Vaccination of children decreases disease in adults. The preponderance of data shows vaccinating against influenza prevents spread to those who are not vaccinated.

The Japanese schoolchildren program provided proof of concept of indirect effectiveness of influenza vaccine. The Central Texas field trial has demonstrated significant herd protection of adults utilizing the live, attenuated influenza vaccine (LAIV) to children. Immunization of <20% of children at the intervention site resulted in an 8-18% reduction of medically attended acute respiratory illness in adults compared to rates in the comparison sites.

When health care workers are vaccinated, patients have decreased mortality and the benefits spill over into the community:

for every 15 healthcare providers who get vaccinated, 1 fewer person in the community will contract an influenza-like illness.

Not bad for protecting yourself and others. I think of the flu vaccine like seat belts. No guarantee that you will not die or get injured in an accident, but just as I would prefer to use seat belts in a head-on collision, I prefer to enter the flu season with a vaccination.

And if you are pregnant?

Vaccination decreases influenza in pregnancy women and their newborns:

Influenza vaccine was immunogenic in HIV-uninfected and HIV-infected pregnant women and provided partial protection against confirmed influenza in both groups of women and in infants who were not exposed to HIV

Decreases the risk of small babies and decreases the spontaneous abortion rates:

Pandemic influenza virus infection in pregnancy was associated with an increased risk of fetal death. Vaccination during pregnancy reduced the risk of an influenza diagnosis. Vaccination itself was not associated with increased fetal mortality and may have reduced the risk of influenza-related fetal death during the pandemic.

Also, by getting the vaccine you can avoid some of the complications of influenza. Death of course, but as those who listen to my ID podcasts know I like to harp on the concept that infections lead to inflammation, inflammation is prothrombotic, and prothrombosis can lead to vascular events, strokes, heart attacks or pulmonary emboli.

Almost every infection that has been evaluated has been found to be associated with a vascular event of one type or another. The difference is that with influenza the vaccine may be protective:

Recent influenza infection was an unrecognised comorbidity in almost 10% of hospital patients. Influenza did not predict AMI, but vaccination was significantly protective but underused.


Evidence from cohort studies and a randomized clinical trial indicates that annual vaccination against seasonal influenza prevents cardiovascular morbidity and all-cause mortality in patients with cardiovascular conditions.

As well as protect against stroke, although the data is variable.

Besides vascular events, influenza vaccination may prevent pneumonia as well.

So as vaccines go, the flu vaccine isn’t perfect but has many benefits at both the individual and population levels. Is it cost effective? Got me. The literature suggests so, and I will take their word for it. Cost effectiveness analyses make my head hurt.

If you like a number needed to treat:

…in well-matched years, if we divide 100 people who got the flu shot by the three people who benefitted, we get a Number Needed to Treat of 33. In well-matched years, for every 33 people who get immunized, one will benefit by not getting a bout of influenza she otherwise would have suffered.
A NNT of 33 is, by the way, a fantastically good therapy. This is far better than the NNT to benefit of many, many other common therapies, like, for example, anti-depressants, anti-hypertensive drugs and prostate cancer screening.


Even in not well-matched years, the flu vaccine is still effective, albeit not quite as well. In not well-matched years, the overall risk of getting the flu drops to 2 people out of 100. The flu vaccine cuts that number in half, for a 50% risk reduction. If you do the math, one person out of 100 benefits from the flu shot, making the NNT in not well-matched years 100.

So those are some of the reasons why you and yours should get the flu vaccine: less chance of flu, less chance of spreading flu, less chance of severe disease, less chance of complications from flu, less chance of death and it makes my Rainbow Unicorn happy.

Prior flu ramblings




  • Mark Crislip, MD has been a practicing Infectious Disease specialist in Portland, Oregon, from 1990 to 2023. He has been voted a US News and World Report best US doctor, best ID doctor in Portland Magazine multiple times, has multiple teaching awards and, most importantly,  the ‘Attending Most Likely To Tell It Like It Is’ by the medical residents at his hospital. His multi-media empire can be found at

Posted by Mark Crislip

Mark Crislip, MD has been a practicing Infectious Disease specialist in Portland, Oregon, from 1990 to 2023. He has been voted a US News and World Report best US doctor, best ID doctor in Portland Magazine multiple times, has multiple teaching awards and, most importantly,  the ‘Attending Most Likely To Tell It Like It Is’ by the medical residents at his hospital. His multi-media empire can be found at