Our guest contributor is Dr. Joshua Goldenberg, a naturopathic physician, professor, researcher, and creator of Dr. Journal Club. He has presented nationally and internationally on evidence-based medicine as well as probiotics and research methodology. He created www.DrJournalClub.com which is an online repository of educational videos serving to keep medical professionals up-to-date on current and relevant research as well as hone their evidence-based medicine skills. In part two of this 4-part series, he focuses on the evidence of the flu vaccine in healthy adults to prevent influenza.
Today we consider a 2014 Cochrane review of randomized controlled trials (RCTs) evaluating influenza vaccine efficacy as well as RCTs and non-RCTs to identify adverse events. RCTs are the best way to identify efficacy because they are the least susceptible study design to bias. However, there may simply not be enough people in an RCT to capture rare adverse events so for harms it may be more appropriate to look at observational studies as well so you aren’t missing rare but important adverse events. The authors found 90 trials that met their inclusion criteria, which had greater than 70,000 participants in just the RCT trials alone. This was a huge data set!
What we see when we crunch the numbers is that for influenza-like illness (caused by influenza and non-influenza viruses) being vaccinated against the flu has a 17% efficacy rate overall so a relative risk of 0.83 when compared to placebo or not getting a shot. This corresponds to a numbers needed to vaccinate of 40. So you would need to vaccinate 40 healthy adults to prevent one case of flu-like illness.
For preventing laboratory proven influenza itself the reduction in risk is 62% and the risk ratio is 0.38. However because the amount of people getting true influenza is much fewer than influenza-like illness overall you need to vaccinate 70 people to prevent one case of true flu. This is based on data from over 51,000 participants.
We see no statistically significant evidence that flu vaccines reduce the numbers of days you are ill, the days you miss from work, the number of prescriptions written, hospitalizations, or complications in these healthy adults. There is one exception in physician visits. Based on a single study if the flu vaccine matched well with the circulating strain of flu virus, the number of physician visits declined.
Local reactions include redness, swelling, and pain in the injection site.. Some people will also get elevated temperature as well as other systemic effects. But there is no evidence of any serious harm with the flu vaccine. It is a safe vaccine.
There are some issues with this study. A major issue is study quality. Less than 10% of the studies had a low risk of bias and more than a quarter of the studies were funded by industry. This is relevant because we know that high risk of bias trials and trials funded by industry are more likely to overestimate effect sizes. So the numbers needed to vaccinate of 40 could be inaccurate (too low). More likely you would need to vaccinate even more people to prevent a single case of flu like illness.
While we see that healthy adults getting vaccinated do not see a decrease in complications or hospitalizations that doesn’t mean that having less infectious adults spreading influenza would not lead to fewer people at risk of complications like the sick and elderly getting the flu. So there is still an argument for herd immunity and protecting vulnerable members of our community.
In summary, you need to vaccinate 40 healthy adults to prevent one case of flu like illness. And there is no evidence that flu vaccination prevents work loss, complications or hospitalizations in these healthy adults. Of course that doesn’t mean it doesn’t prevent them in more at risk populations. There are local harms such as tenderness and pain in the injection site but there is no evidence of serious harms from the flu vaccine.