# 6819
It happens every flu season, but this year, it seems to be getting more attention than usual; reports of people who got this year’s flu vaccine, but still ended up catching the flu.
One such story appeared overnight on WTVA-TV, out of Tupelo-Columbus Mississippi, called Flu season a bit different this year (h/t Emily on FluTrackers) where a local doctor speculates:
"Well, the flu season has been fascinating because we thought we had a vaccine this year that was the best we ever had for the flu we were seeing early in the flu season. But now, it looks like there is another strain that's not in the vaccine. And that might be the reason we've had such an explosion of flu cases in Mississippi," said Dr. Edward Hill.
While a `new strain’ is always a possibility, at best, the flu vaccine is only expected to protect 50%-70% of healthy adults who take it (see Lancet: Low Flu Vaccine Effectiveness) – and is likely even less effective for the elderly or those with compromised immune systems.
So a certain number of individual `vaccine failures’ are to be expected, even when the flu vaccine is a good match to the strains in circulation.
Still, whenever we start seeing reports like this the subject of a `new or mutated strain’ of flu always comes up.
Flu viruses are very good at mutating and evolving. NIAID has a nice 3-minute video illustrating how viruses change over time, which you can view on their Youtube Channel or in the box below.
So it is always a bit of a crapshoot when flu experts gather twice each year to select the flu strains to included in the flu vaccine that will be delivered 6 months later.
Yesterday, the CDC updated their FAQ on What You Should Know for the 2012-2013 Influenza Season, where they address recent reports of people getting the flu despite getting the vaccine.
Has CDC received reports of people who have gotten a flu vaccine and then tested positive for influenza?
Yes. CDC has received reports of some people who were vaccinated against influenza becoming ill and testing positive for influenza. This occurs every season. It’s not possible at this time to say whether or not there is more of this happening this season than usual. This is an early season, with more influenza activity being reported at this time than has been seen during recent flu seasons. CDC is watching the situation closely and will provide additional information, including interim vaccine effectiveness (VE) estimates, as it becomes available. There are, however, a number of reasons why people who got an influenza vaccine may still get influenza this season:
- People may be exposed to an influenza virus shortly before getting vaccinated or during the two-week period that it takes the body to gain protection after getting vaccinated. This exposure may result in a person becoming ill with flu before the vaccine begins to protect them.
- A person may be exposed to an influenza virus that is not included in the seasonal flu vaccine. There are many different influenza viruses that circulate every year. The composition of the flu shot is reviewed each season and updated if needed to protect against the 3 viruses that research suggests will be most common. Characterization of influenza viruses collected this season in the United States indicates that most circulating viruses are like the vaccine viruses however, there is a smaller percentage of viruses that would not be expected to be covered by the vaccine.
- Unfortunately, some people can get infected with an influenza virus that is included in the vaccine despite getting vaccinated. Protection provided by influenza vaccination can vary widely, based in part on health and age factors of the person getting vaccinated. In general, the flu vaccine works best among young healthy adults and older children. Some older people and people with certain chronic illnesses may develop less immunity after vaccination. While vaccination offers the best protection against influenza infection, it's still possible that some people may become ill after being vaccinated. Influenza vaccination is not a perfect tool, but it is the best tool currently at our disposal to prevent influenza.
To estimate how well influenza vaccines work each year, CDC has been working with researchers at universities and hospitals since the 2003-2004 influenza season conducting observational studies using laboratory-confirmed influenza as the outcome. At this time, there is insufficient data to provide estimates about VE for this season. CDC hopes to have interim VE estimates within the next 5 weeks. These estimates will provide more information about how well this season’s vaccine is working.
We can really only judge the effectiveness of each year’s flu vaccine in retrospect, and often not until the flu season has ended. The viruses that circulate are a constantly moving target, and even a small change to the virus can have a major impact on how well the vaccine works.
The good news is, there is evidence to suggest that even in years when the vaccine isn’t a particularly good match, getting the shot may help reduce the severity of one’s influenza illness. Again from the CDC.
Can the vaccine provide protection even if the vaccine is not a "good" match?
Yes, antibodies made in response to vaccination with one flu virus can sometimes provide protection against different but related viruses. A less than ideal match may result in reduced vaccine effectiveness against the virus that is different from what is in the vaccine, but it can still provide some protection against influenza illness.
In addition, it's important to remember that the flu vaccine contains three virus viruses so that even when there is a less than ideal match or lower effectiveness against one virus, the vaccine may protect against the other viruses.
While most years flu vaccines do provide a moderate level of protection, in truth, we need better, more protective vaccines that work well for everyone, not just healthy adults.
For more on this see:
CIDRAP: The Need For `Game Changing’ Flu Vaccines
