As protection goes, the flu shot is no smallpox vaccine – millions of people get sick even after getting their annual injection. Some year's influenza vaccines have provided better protection than others, but a study has shown that even the least effective rounds save astonishing numbers of lives, and vast amounts of pain and lost income.
The flu virus is always changing, rapidly evolving and shuffling surface proteins to evade our defenses. Although work is underway towards a universal flu vaccine, we may be waiting a long time. Meanwhile, the disease kills 12,000-56,000 people in the United States alone. Worldwide the figure is many times that, and the threat of a repeat of the 1918 epidemic that killed more than 50 million people haunts epidemiologists' nightmares.
There are always many different strains of the virus in circulation. The vaccine only protects against the three expected, not always accurately, to be the most widespread that year, and sometimes imperfectly even against them. On average, flu shots have been found to provide 45 percent protection, but in 2014-15 this was just 19 percent.
Consequently, many people assume getting the flu shot just isn't worth it, but while individual circumstances vary, a paper in Proceedings of the National Academy of Sciences suggests they should think again.
The authors, led by Yale University's Dr Pratha Sah looked a the consequences of a 20 percent effective flu shot, based on 43 percent of the American public getting vaccinated – the long-term average. It found these injections prevent 21 million infections, 130,000 hospitalizations and 62,000 deaths, just in America. Remember that is in a year where the protectiveness of the vaccine is below average, in other years the numbers are even higher. Most people choose to get vaccinated, or not, before the protective value that year is known. Getting vaccination rates to 50 percent, even without improving vaccine quality, would save another 8,000 lives. Even if you don't get vaccinated for yourself, do it for others.
The numbers are so high because, when it works, the vaccine does not just protect those who get it, but provides herd immunity. Every person with flu has the potential to spread it to others, and by stopping some people from getting infected, the vaccine breaks up the lines of transmission, saving even those who never got the shot, or for whom the vaccine was not directly protective.
The benefits can be maximized by focusing vaccinations on specific age-groups, but curiously when the authors modeled different scenarios, they found this changes with effectiveness. When the vaccine's efficacy is low, it's best given to the elderly, who are most at risk. In years when its protection is higher, its greatest potential lies in stopping children and young adults from spreading the disease to those more vulnerable.