How health officials determine the viruses used in the flu vaccine

A new report form the Centers for Disease Control and Prevention (CDC) has revealed this year’s flu season has been getting worse – especially for the elderly.

Since the CDC’s previous report, nine children have died of the influenza-associated illnesses – bringing the total number of pediatric deaths to 29.

While the severity of the flu season may be on the rise, it’s never too late to arm yourself against by getting vaccinated.  Fortunately, CDC officials have touted this year’s flu vaccine to be an excellent match against the most prominent H3N2 strain of the flu circulating this season.

But how do researchers know which flu strain will be the one to cause the most damage each year? According to Dr. Nancy Cox, director of the Influenza Division at the CDC, researchers are constantly on the lookout for indications that there are different viruses circulating than there are in the influenza vaccine.  In order to understand if any new, more dangerous strains are in circulation, CDC officials have to work with numerous countries from all across the globe.

“We are receiving viruses from all over the world, testing those viruses to see if they are similar or different to what’s already in the vaccine,” Cox, who is also director of the CDC’s World Health Organization (WHO) Collaborating Center for Surveillance, Epidemiology and Control of Influenza, told  “If we do see a new variant virus, we then look at the spread of that virus within China or other countries.  If we see that pattern, we take that virus and test it against serum from people who have been vaccinated to see if they have antibodies against that new virus.”

If the antibodies people produce are at lower levels, researchers know it is time to update the vaccine.

Once they have analyzed the way current strains have been spreading and how the current vaccine responds to what’s circulating, the CDC uses all of the new information to make a recommendation about which viruses should be used in the current vaccine.

“A group of experts, including the CDC experts, work under the umbrella of the World Health Organization (WHO),” Cox said.  “So we participate twice a year in a consultation that makes recommendations for what components will be in vaccines for the northern hemisphere and for the southern hemisphere. We have a lot of data that we gather and analyze and present at that meeting and so we’re an important [component].”

Once the final recommendations are made for the flu vaccine by WHO, the FDA’s Vaccines and Related Biological Products Advisory Committee reviews the decision and decides which viruses the U.S. will use.  The FDA typically decides what WHO decides, but it ultimately provides a system of checks and balances so that the best vaccine is produced in the United States.

The current flu vaccines is made up of three components of influenza virus – one influenza A (H3N2) virus, one seasonal influenza A (H1N1) virus and one influenza B virus.  These components reflect the kinds of viruses circulating throughout the globe.

“There was a period of time where we only had one A and one B circulating,” Cox said of the changing strains.  “Now we have two As and two Bs circulating around the globe…Within those groups, you can have a couple of sub-groups in a given year.  A new variant emerging in one of the A groups is totally dependent on the season.  What circulates is dependent on changes in the virus, immunity in the population, etc.”

Cox noted it is hard to determine exactly how many strains there are in total, as the virus is constantly evolving and changing.  However, she maintained that this year’s flu vaccine is extremely well-matched to the main flu viruses circulating this winter.  The vaccine is extremely well- matched to the H3N2 strain, which is the most prominent strain circulating this season, as well as the H1N1 circulating, too.

As for the B component, Cox said the vaccine matches around two-thirds of the strains that are circulating, not accounting for the other third.  But since 70 percent or more of the viruses wreaking havoc this season are caused by the influenza A viruses, this year’s vaccine is extremely protective.

However, those who get the vaccine will never have a 100 percent guarantee that they won’t get sick.

“Even with a good match year, some people still don’t mount a good antibody response to the vaccine,” Cox said. “So even if the vaccine components match what’s circulating, you’ll have some vaccine breakthroughs.”

No matter what, there is absolutely no way to be certain about which flu strains will circulate – or if completely new strains will emerge.

“There’s no predictable pattern for seasonal viruses to change and no internal clock or pattern that you can use to predict when the next pandemic virus will emerge,” Cox said.  She noted the most major pandemics in which new strains emerged occurred in 1918, 1957, 1968; along with the re-emergence of H1N1 in 1971 and again in 2009 – making the pattern completely unpredictable.

“It’s a yearlong job,” Cox said of monitoring, picking and choosing what goes into the flu vaccine. “It’s sort of high-risk, high-stress for everyone involved.”