Flu Vaccine a Global Public Health Triumph Despite “Only” 59% Effectiveness

Every year public health officials, doctors and advertisements outside of Walgreen’s remind us to get our flu vaccines. But a recent report in the Lancet Infectious Disease showed that the flu vaccine is only 59% effective, on average. While 59% might sound paltry, considering the challenges that must be surmounted on a yearly basis, flu vaccines are actually a huge unrecognized global public health success. This triumph has roots in both the epidemiology of flu and the surprising degree of consistent international coordination required to overcome influenza’s unique biological challenges.

Some diseases, like polio or smallpox, can be prevented with one vaccine. With that one shot, our immune systems can recognize and destroy those viruses usually for the rest of our lives.

We’re not so lucky with influenza. Circulating influenza virus often changes its surface antigens over the course of a year (a process known as reassortment). A changing virus means we need to re-train our immune systems to fight off a new influenza virus every year. As a result, we have at least a 5 to 20% chance of getting the flu each winter, even if we’ve been sick before or gotten the prior year’s vaccine. (If the new flu virus is similar to one we’ve fought off before, we do have some immunological protection, but we’re still vulnerable to infection.)

Influenza’s virology thus creates policy challenges. To make a new vaccine, we need to know something about how the influenza virus is changing as it infects birds, pigs and people in northern and southern hemispheres. That’s only possible if countries around the globe analyze their influenza viruses and share that information with all other countries. When that sharing happens, scientists can use that data to choose which influenza viruses should be included in the yearly vaccine.

Thankfully, the World Health Organization has sheparded just such a system since 1947. The Global Influenza Surveillance Network (GISN) is a network of laboratories that share information about how the influenza virus is changing as it spreads around the globe. Since the scientists in GISN knew the most about the latest influenza viruses, in 1971, the WHO decided that this group of people should have a meeting every year to recommend which influenza virus should be made into the vaccine. Every February, scientists gather in Geneva to choose the three strains of the virus most likely to be causing flu during the next flu season in the northern hemisphere. In September, they do the same for the southern hemisphere.

Keep in mind that the scientists are making their best guess about which flu viruses might be circulating over six months in the future. They have to choose so far in advance because vaccine manufacturers need plenty of lead-time to make the vaccine by flu season.  As a result, sometimes the scientists get it right and the flu vaccine prevents a lot of flu infections. Other times, their guess is imperfect. In those instances, the vaccine does not do as well in protecting us from flu infections. On average, the Lancet ID study showed, scientists have guessed well enough for 59% efficacy.

From modest beginnings of only a handful of participating laboratories and countries, GISN now has five collaborating centers, 136 National Influenza Centers and 106 countries participating in the Network. On a yearly basis, these Centers share information on circulating influenza viruses, a group of experts decide which of those virus strains to include in the vaccine, and pharmaceutical companies create the vaccine. This happens like clockwork every year in order to bring us the flu vaccine. It’s a system often taken for granted* but when it works well it is a surprising example of international coordination at its finest.

* Indonesia briefly boycotted GISN in 2005 due to concerns about intellectual property and access to vaccines resulting from H5N1 (“bird flu”) viruses it shared with the network. It has since resumed sharing with GISN and a WHO agreement has been reached. More on this in a future Disease Daily report.

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