The Science of the Flu Shot

Arriving with the autumnal chill in the air, flu shots are being dispensed in clinics and pharmacies across the country. While imperfect, the vaccination is the most reliable way to avoid a potentially deadly infection. While many accept it as a seasonal inconvenience, influenza kills about 19,000 Americans in an average year.

Following the pioneering work of Hilary Koprowski, who developed a vaccination protocol using live-attenuated virus, Jonas Salk and Thomas Francis created the world’s first modern flu vaccine in 1938 using fertilized chicken eggs. Their vaccine was first used to protect American troops during World War II. In the years since, the method to produce the vaccine has largely remained the same, but the quality of information that goes into its development has changed dramatically, says Stephen S. Morse, an infectious disease expert and professor of Epidemiology.

Influenza is now known to be in a constant state of flux, mutating as it passes from one disease reservoir to another—from migratory birds to chickens to humans and back. It also comes in different variations: influenza A, B, C, and D. The first two are the most common and are the ones used in the vaccine.

In the early years, scientists created the vaccine based on much less data. Today they predict what strains will appear in the United States and other Northern Hemisphere countries by using a combination of methods, including using data collected from large samples in the Southern Hemisphere where the flu season peaks during our summer months.

Flu shots today contain three or four strains of influenza A and B that scientists think are most likely to spread in the month ahead. Even if the stain isn’t an exact match, it can convey partial protection. For all the advances, however, influenza strain selection is still an inexact science.

The effectiveness of a flu shot can vary widely. In 2014-15, the vaccine was estimated to work for only 23 percent of people, but in the previous year it was 51 percent.

“What we do, is partly art and part science,” says Morse. “The flu remains one of the more complex viruses we deal with, and no one can tell you with certainty what this year’s strain will be until it appears. It seems so prosaic, so mundane, and yet it defies all our attempts at prediction.”

Jeffrey Shaman, an associate professor in Environmental Health Sciences, has adapted techniques used in weather forecasting to produce real-time, location-based estimates of flu outbreaks. Even though the technology is not used in strain selection, the forecast can promote greater vaccination among the public. For health officials, it can inform decisions on how many vaccines and antiviral drugs to stockpile, and in the case of a virulent outbreak, whether other measures, like closing schools, are necessary.

“Flu never surprises me,” says Melissa Stockwell, associate professor of Population and Family Health and Pediatrics. “It’s also a tricky virus, and unlike other vaccinations where we can get one series of doses and have prolonged protection, we have to get a new dose each year.”

Stockwell’s recent work has been in adapting new technologies to survey respiratory infections, educate people in treatment, and remind them to get their shots. She found that more than a quarter of people with the flu don’t visit a doctor for treatment when they show symptoms. The good news is that educational text messages can increase the chances someone will get a flu shot.

She believes that misconceptions about the flu shot, such as the vaccine making recipients sick, are keeping more people from getting vaccinated. Boosting the image of getting a flu shot through social media, she thinks, might be a solution to make getting a flu shot more popular. One way to do this, she says, would be by adapting the “I Voted” campaign to say “I Got My Flu Shot” to make it something people can feel proud of.

Because, while alternatives like hand sanitation and avoiding sick people may be somewhat effective in preventing infection, nothing works like a flu shot.

“You can’t avoid sitting on the train and having someone suddenly cough on you,” says Stockwell. “The vaccine is really the single best way to avoid getting sick.”