Did a scientific study conclude that vitamin D supplements are more effective than flu vaccines in preventing influenza? No, that's not true: Stories making the claim cite reviews and studies that concluded there are significant benefits from maintaining a good level of Vitamin D in your system for the prevention of upper respiratory tract infections (such as colds), but only one of the cited vitamin D studies focused on influenza prevention (or even reference the flu.)
The story originated from an article written by Dr. Joseph Mercola, who sells Vitamin D supplements, pubished on September 24, 2019 titled "Study: Vitamin D Is More Effective Than Flu Vaccine" (archived here) which opened:
Conventional health authorities claim getting a flu shot each year is the best way to ward off influenza. But where's the actual science backing up that claim?
If you've repeatedly fallen for this annual propaganda campaign, you may be surprised to find the medical literature suggests vitamin D may actually be a FAR more effective strategy, and the evidence for this goes back at least a decade.
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Study: Vitamin D Is More Effective Than Flu Vaccine
Conventional health authorities claim getting a flu shot each year is the best way to ward off influenza. But where's the actual science backing up that claim? If you've repeatedly fallen for this
The article then quoted a doctor who wrote about his observations as a psychiatrist in a hospital for the criminally insane. Dr. Cannell did based his conclusions about the benefits of vitamin D on research, but only on what happened during a flu outbreak in the hospital:
Dr. John Cannell, founder of the Vitamin D Council, was one of the first to introduce the idea that vitamin D deficiency may actually be an underlying CAUSE of influenza.His hypothesis  was initially published in the journal Epidemiology and Infection in 2006.  It was subsequently followed up with another study published in the Virology Journal in 2008. 
The first citation is from Dr. Cannell's article titled "REVIEW ARTICLE -- Epidemic influenza and vitamin D" which concluded:
There is much evidence to suggest that vitamin D may be Hope-Simpson's seasonal stimulus. Nevertheless, it is premature to recommend vitamin D for either the prevention or treatment of viral respiratory infections. It is not, however, too early to recommend that health-care providers aggressively diagnose and adequately treat vitamin D deficiency.
The second citation is from a paper (not a study) by Dr. Cannell titled "On the epidemiology of influenza" that explains his hypothesis that vitamin D could help prevent influenza. Again, this is not a "study" as claimed in the article.
The article then references a 2009 study (yes, an actual published study) titled "Association Between Serum 25-Hydroxyvitamin D Level and Upper Respiratory Tract Infection in the Third National Health and Nutrition Examination Survey":
The following year, the largest nationally representative study  of its kind to date discovered that people with the lowest vitamin D levels indeed reported having significantly more colds or cases of the flu. In conclusion, lead author Dr. Adit Ginde stated:
"The findings of our study support an important role for vitamin D in prevention of common respiratory infections, such as colds and the flu. Individuals with common lung diseases, such as asthma or emphysema, may be particularly susceptible to respiratory infections from vitamin D deficiency."
But that quote is NOT in the study. The researchers focused on "the common cold" -- not influenza and vitamin D. The quote Dr. Ginde is from a news release issued by a hospital.
Upper respiratory tract infection (URTI), or the "common cold," is the most widespread infectious disease and the most common reason for US emergency department visits and unscheduled outpatient visits.1 More than 200 viruses contribute to the clinical syndrome of cough, nasal congestion, nasal discharge, sore throat, and sneezing.2,3 Most adults in the United States experience 2 to 4 URTIs per year, and most children experience 6 to 10 per year, which carries enormous population morbidity due to the high incidence of disease and disruption caused by symptoms.4,5
There are major distinctions between the common cold and the flu, according to Healthline.com's definition of accute upper respiratory infection:
What is acute upper respiratory infection?
Anyone who has ever had a cold knows about acute respiratory infections (URIs). An acute URI is a contagious infection of your upper respiratory tract. Your upper respiratory tract includes the nose, throat, pharynx, larynx, and bronchi.
Without a doubt, the common cold is the most well-known URI. Other types of URIs include sinusitis, pharyngitis, epiglottitis, and tracheobronchitis. Influenza, on the other hand, isn't an URI because it's a systemic illness.
This article next cited a 2017 meta-analysis (not a study) of 25 published studies into the protective benefits of vitamin D supplementation in relation to upper respiratory infections:
Since then, a number of studies have come to similar conclusions. Most recently, a scientific review [5,6] of 25 randomized controlled trials confirmed that vitamin D supplementation boosts immunity and cuts rates of cold and flu.
This review titled "Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data". The analysis concluded that people who had good levels of vitamin D already reduced their risks of an upper respiratory infection by about 10 percent if they took daily or weekly vitamin D supplements. Those with the most significant deficiency of vitamin D gained about a 50 percent decreased risk:
25 eligible randomised controlled trials (total 11 321 participants, aged 0 to 95 years) were identified. IPD were obtained for 10 933 (96.6%) participants. Vitamin D supplementation reduced the risk of acute respiratory tract infection among all participants (adjusted odds ratio 0.88, 95% confidence interval 0.81 to 0.96; P for heterogeneity <0.001). In subgroup analysis, protective effects were seen in those receiving daily or weekly vitamin D without additional bolus doses (adjusted odds ratio 0.81, 0.72 to 0.91) but not in those receiving one or more bolus doses (adjusted odds ratio 0.97, 0.86 to 1.10; P for interaction=0.05). Among those receiving daily or weekly vitamin D, protective effects were stronger in those with baseline 25-hydroxyvitamin D levels <25 nmol/L (adjusted odds ratio 0.30, 0.17 to 0.53) than in those with baseline 25-hydroxyvitamin D levels ≥25 nmol/L (adjusted odds ratio 0.75, 0.60 to 0.95; P for interaction=0.006). Vitamin D did not influence the proportion of participants experiencing at least one serious adverse event (adjusted odds ratio 0.98, 0.80 to 1.20, P=0.83). The body of evidence contributing to these analyses was assessed as being of high quality.
But this review did not focus on influenza. It acknowledged a varying range of definitions of upper respiratory infections among the 25 separate studies.
The article does point to a small study in 2010 involving Japanese school children that suggested daily doses of vitamin D during flu season could reduce the risk of influenza. The study, titled "Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren":
Results: Influenza A occurred in 18 of 167 (10.8%) children in the vitamin D3 group compared with 31 of 167 (18.6%) children in the placebo group [relative risk (RR), 0.58; 95% CI: 0.34, 0.99; P = 0.04]. The reduction in influenza A was more prominent in children who had not been taking other vitamin D supplements (RR: 0.36; 95% CI: 0.17, 0.79; P = 0.006) and who started nursery school after age 3 y (RR: 0.36; 95% CI: 0.17, 0.78; P = 0.005). In children with a previous diagnosis of asthma, asthma attacks as a secondary outcome occurred in 2 children receiving vitamin D3 compared with 12 children receiving placebo (RR: 0.17; 95% CI: 0.04, 0.73; P = 0.006).
The study, which was limited to four months and 334 children, suggested "vitamin D3 supplementation during the winter may reduce the incidence of influenza A", it showed no benefits for Influenza B. These researchers noted in their analysis that "no rigorously designed clinical trials have evaluated the relation between vitamin D and physician-diagnosed influenza," and they pointed to another study of adults that "could not reconfirm the benefit of vitamin D supplementation for the prevention of symptomatic upper respiratory tract infections." That study, titled "A randomized controlled trial of vitamin D3 supplementation for the prevention of symptomatic upper respiratory tract infections", found no difference in vitamin D and a placebo in the incidences of upper respiratory infections during the winter flu season:
To determine whether vitamin D supplementation during the winter season prevents or decreases URI symptoms, 162 adults were randomized to receive 50 μg vitamin D3 (2000 IU) daily or matching placebo for 12 weeks. A bi-weekly questionnaire was used to record the incidence and severity of URI symptoms. There was no difference in the incidence of URIs between the vitamin D and placebo groups (48 URIs vs. 50 URIs, respectively, P=0·57). There was no difference in the duration or severity of URI symptoms between the vitamin D and placebo groups [5·4±4·8 days vs. 5·3±3·1 days, respectively, P=0·86 (95% CI for the difference in duration −1·8 to 2·1)].
So, several papers and studies provide reasons to take vitamin D supplements to reduce the risks of colds -- and maybe even influenza. But the article claims that a "study" found vitamin D to be more effective that a flu vaccine. This requires an answer to the question: How effective are flu vaccines in preventing the flu?
The questionable article claims "flu shots don't work." It points to a 2010 review of previous studies that it claims made a "bombshell conclusion" that has been "ignored by mainstream media" The review titled "Vaccines for preventing influenza in healthy adults" was likely ignored because it had no bombshell and it is nine years old. Instead, let's look at a 2019 review on the same site titled "Is the influenza vaccine effective in decreasing infection, hospitalization, pneumonia, and mortality in healthy adults?" It concluded that researched showed between a 16% and 59% efficacy rate. The 16% rate applied to suspected flu cases not confirmed by a lab test, so the number of flu cases could be inflated. The 59% rate included infuenza cases confirmed by a lab test:
The evidence from four systematic reviews indicates inactivated influenza vaccination efficacy of between 16-59% in healthy adults based on whether the results are clinically derived or empirically derived.
The misleading article also claims that medical reviews showed "negative findings" about the ability of flu vaccines to prevent the illness in children. It pointed to a 2006 review titled "Vaccines for preventing influenza in healthy children" which it claimed "found no evidence that the flu vaccine is any more effective than a placebo in children under two." In fact, the review did not say it found "no evidence" that thvaccines were not effective in children under two. It only noted that it was "surprising" there was only one study into the question available. As for children three and older, the review concluded flu vaccines were helpful:
Influenza vaccines are efficacious in children older than two years but little evidence is available for children under two. There was a marked difference between vaccine efficacy and effectiveness. That no safety comparisons could be carried out emphasizes the need for standardisation of methods and presentation of vaccine safety data in future studies. It was surprising to find only one study of inactivated vaccine in children under two years, given recent recommendations to vaccinate healthy children from six months old in the USA and Canada. If immunisation in children is to be recommended as public-health policy, large-scale studies assessing important outcomes and directly comparing vaccine types are urgently required.
Vaccines change each year as scientists adjust them to address changes detected in the influenza virus. It is a guessing game -- although an educated guess -- that sometimes fails to match what emerges in a flu season. Getting a flu shot is no guarantee that you won't get the flu.
Yes, taking vitamin D does seem to be a great idea, especially in the winter months when you might get less of it naturally from the sun.
But we concluded there is NO study that found vitamin D is more effective than a flu vaccine.
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