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Posted by Donna Lazorik, RN, MS. Donna is the Immunization Coordinator in the Division of Epidemiology and Immunization at the Massachusetts Department of Public Health.

Over the past few weeks, DPH has received a lot of questions concerning how well this year’s vaccine matches the kinds (or “strains”) of flu that are currently circulating, and why it might be that vaccinated people can still get the flu.  

Every year, some people who are vaccinated against influenza still get sick and test positive for flu.  It’s not possible at this time to say whether or not there is more of this happening this season than usual.  What we do know is that this is an early season, with more influenza activity being reported at this time than has been seen during recent flu seasons.  DPH and the U.S. Centers for Disease Control and Prevention (CDC) are watching the situation closely and will provide additional information as it becomes available. 

There are, however, a number of reasons why people who got a flu vaccine may still get sick this season:

1. 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 getting the flu before the vaccine begins to protect them.

2. 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, if necessary, updated to protect against the 3 viruses that research suggests will be most common. Studies of the flu viruses collected this season indicate that most circulating viruses are like the vaccine viruses (a good match).  However, there is a smaller percentage of viruses that would not be expected to be covered by the vaccine.  

Each year, DPH sends a small sample of flu specimens tested at the State Laboratory to the CDC so they can identify the flu viruses and determine whether they are consistent with what is included in this season’s vaccine.  So far this season, CDC has tested three specimens from Massachusetts and all three were viruses included in the vaccine.  Nationally, 91% of the samples tested by CDC have been consistent with viruses included in the vaccine.

3. Unfortunately, some people can get infected with an influenza virus that is included in the vaccine even if they have been vaccinated.  Protection provided by flu vaccination can vary widely, based in part on the health and age of the person getting vaccinated.  In general, the flu vaccine works best among young healthy adults and older children, although it does not provide 100% protection even among these groups.  Some older people and people with certain chronic illnesses may develop less immunity after vaccination.  However, even in people that do not respond as well to the vaccine, the flu vaccine still provides some level of protection, and may result in less severe illness if they become sick with the flu.

All that being said, vaccination is still the best tool currently available to prevent the flu. If you have not already been vaccinated, it still makes sense to get the flu vaccine.  In Massachusetts, flu season usually peaks in February, and continues through April and May.  Contact your healthcare provider to schedule an appointment, or go to to find a clinic near you.

For more information about the flu, visit

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