WHO’s in Charge: How the CDC and WHO Prepare Vaccines for Flu Season

Every New Year brings the inevitable flu season. With fingers crossed, those of us who received flu shots in the fall remain hopeful that we’ll be protected during the winter months from any one of the several influenza viruses circulating at work, school, and other crowded spaces. Sometimes we’re lucky, sometimes we’re not.

Have you ever wondered how flu vaccines are formulated in the first place? Or which agencies are involved? Or how they determine which viruses to include for inoculation? And what about the big question people are asking these days – does a flu vaccine really work? Let’s explore these questions and more.

The Mission of CDC’s Influenza Division

You might have guessed that the Centers for Disease Control and Prevention (CDC) take primary responsibility for flu readiness in the United States. Or as the CDC website states authoritatively: “Globally coordinated epidemiologic and virologic surveillance is the foundation of the influenza vaccine virus selection and development process and is a critical component of pandemic preparedness.”

Located in Atlanta and in operation since 1956, the CDC Influenza Division acts as one of five World Health Organization (WHO) Collaborating Centers (CC) for Surveillance, Epidemiology, and Control of Influenza. As the largest center in the world, its team of scientists plays a leading role in year-round surveillance for early identification of seasonal influenza viruses that may pose the greatest global threat.

Among its many public health functions, the CDC Influenza Division:

• Reports and documents information on influenza activity in the U.S. each and every week
• Collects and analyzes influenza viruses for their epidemiological, genetic, and immune response characteristics
• Makes recommendations on appropriate viruses to be included in annual seasonal influenza vaccines
• Creates candidate vaccine viruses, which are prepared flu viruses used by vaccine manufacturers to mass produce flu vaccines

The Global Influenza Surveillance and Response System (GISRS)

The trouble with “the flu” is that there isn’t just one. We’ve all heard a lot about some of the more notorious strains, such as Spanish, swine, and avian flu. In fact, there are many more – too many for any single vaccine to cover. Instead, the World Health Organization determines the three or four viruses that scientific research indicates are most likely to cause serious illness and spread among the global population for the upcoming flu season.

It’s an ongoing process. Week by week, national influenza centers in more than 100 countries conduct surveillance of flu virus samples obtained from thousands of patients. These laboratories send their findings to the CDC in Atlanta, as well as to four other participating WHO Collaborating Centers:

• The Francis Crick Institute in London
• Victoria Infectious Diseases Reference Laboratory in Melbourne
• National Institute for Infectious Diseases in Tokyo
• National Institute for Viral Disease Control and Prevention in Beijing

Twice each year, WHO organizes a Global Influenza Surveillance and Response System (GISRS) meeting with the directors from each center. Together, they make recommendations about the viruses to be included in the vaccines for the flu seasons in both the Northern and Southern Hemispheres. Each country then uses the GISRS report to decide which viruses to include in their own nation’s licensed vaccines. In the U.S., the FDA makes that final decision.

Determining the Best Vaccine Candidates

The flu viruses used for the seasonal vaccine are selected annually based on data indicating which are currently circulating, combined with forecasts about which are the most likely to circulate during the upcoming flu season. Another important practical factor is whether a vaccine virus that is similar to a virus in circulation is currently available.

Timing is paramount. Vaccine viruses must be tested with enough lead time to allow for their mass cultivation necessary to provide a global supply of vaccine. Sometimes a suitable vaccine virus cannot be identified or developed to meet the production deadline, explaining why vaccines may not always inoculate against all potential flu viruses that circulate each season.

At the same time, certain influenza viruses may not appear and spread until later in the season, making it difficult to prepare a candidate vaccine virus in time for vaccine production. So, even with the most accurate scientific methods, neither WHO nor CDC cannot completely anticipate how a flu pandemic may run its course.

As soon as WHO makes its recommendation, the private sector begins the process of producing large quantities of influenza vaccine, which can take more than six months. For the vaccine to be delivered in time for inoculations to begin in the fall, manufacturers often start growing one or more of the vaccine viruses in January based on their best guess as to what viruses are most likely to be included in the vaccine.

Do vaccines really work?

In recent years, a growing number of people have become suspicious of flu vaccines, most often for dubious reasons based on false information spread through social media. Even so, a good percentage of the public forgoes getting a flu shot for a more prosaic reason: they simply don’t believe it works. We all know someone who insists that they never got the flu until they got a flu shot. You might be one of them.

We’ve already seen that there’s a good reason why that could happen. But what, really, are the odds? Is a flu vaccine worth the time, money, and needle prick? What’s the evidence?

CDC conducts studies each year to determine how well the vaccine protects against the flu illness. Its most recent findings indicate that flu vaccination reduces the risk of flu illness by between 40% and 60% among the overall population. During years when the flu vaccine is not well matched to circulating influenza viruses, it’s possible that little or no benefit from flu vaccination may be observed. During years when there is a good match between the flu vaccine and circulating viruses, flu vaccination can offer substantial benefits in terms of preventing flu illness and complications.

It should not, therefore, come as a surprise that vaccination effectiveness varies from season to season. Equally significant, even during years when the flu vaccine match is good, the benefits of flu vaccination will vary, depending on factors such as the age and health of the person being vaccinated, the virulence of the viruses circulating that season, and even, potentially, which type of flu vaccine was administered.

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