Worth a shot? Just how effective is the flu vaccine?

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Science Explainer: What You Should Know About This Year's Flu

Q: Is this year's flu season really worse than normal?

A: Maybe. But if the flu seems to be everywhere right now, it's probably just because it's off to an early start. In the United States, data from the Centers for Disease Control and Prevention (CDC) in Atlanta seem to suggest that the number of cases peaked in late December, although the agency warns that numbers for recent weeks may still climb as more reports come in. In contrast, last year's flu season peaked in March. In Europe, too, flu seems to have come early; 10 countries, most of them in northwestern Europe, already report "widespread" activity, according to the latest update from the European Centre for Disease Prevention and Control (ECDC) in Stockholm.

Q: Which influenza strains are circulating at the moment?

A: In every influenza season, there are three so-called subtypes of the virus: H1N1 and H3N2 (both influenza A viruses) and influenza B. But which of the three dominates can vary by country, state, or even by region. In Europe, more than half of all specimens characterized so far have been influenza B; the two influenza A subtypes each represent about a quarter of specimens. In the United States, influenza B is far less prominent at the moment, and H3N2 dominates. Although H3N2 tends to be associated with increased sickness and death, U.S. mortality rates aren't substantially higher than usual so far, according to CDC.

Q: How do scientists choose the strains to put in the vaccine? Is this year a good match?

A: To offer maximum protection, flu vaccines always contain all three subtypes. Every year, a group of experts convened by the World Health Organization (WHO) makes recommendations about which strains of each subtype to include in next winter's vaccine. (Their advice for the Northern Hemisphere usually comes in February, to give vaccine producers enough time to produce the new vaccine.) Although WHO monitors global influenza activity year-round to determine which strains are likely to dominate a given flu season, it's still virtually impossible to predict a season's severity in advance.

So far, this year's vaccine is a good match to the strains found in Europe, according to ECDC, meaning it elicits antibodies to the viruses that actually circulate. However, a good match does not necessarily mean good protection; the relationship between the two is not entirely clear. Early data from CDC suggests this year's vaccine is only about 60% effective.

Q: Why is the current vaccine only 60% effective?

A: Today's flu vaccines work by causing the body to develop antibodies to hemagglutinin (HA), a protein that covers the surface of the influenza virus. But because it mutates easily, the exact sequence of HA varies from strain to strain. That means no HA-head antigen vaccine can protect against multiple strains of influenza, leaving even people who were vaccinated vulnerable to infection by slightly different versions of the virus. A recent study by a group at the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota concluded that from 1967 to 2012, the flu vaccine was only 59% effective—significantly lower than the protection offered by most routinely recommended vaccines. In order to substantially improve protection against the flu, the report concluded, researchers need to create a vaccine that targets antigens shared by multiple strains of the virus, such as the HA stalk, nucleoprotein, and the matrix 2 protein.

Q: What are the commercial and academic hurdles to developing a better vaccine?

A: As Nicholas Kelley, research associate at CIDRAP and a key author of the report, explained in a recent ScienceLIVE chat, "[O]ne of the greatest challenges facing the development of new novel-antigen, game-changing influenza vaccines is the perception that our current vaccines are good enough." Indeed, most public health policy related to the flu vaccine focuses on incremental improvements such as vaccinating more people or manufacturing existing vaccines more quickly. Programs aimed at developing novel-antigen or universal flu vaccines receive little federal or international support, Kelley says. And because bringing even a relatively simple new vaccine to market can cost more than $1 billion and take up to 15 years, private companies don't want to take the financial risks associated with developing a radically new vaccine on their own.