The CDC say that this year's flu shot is likely to reduce the risk of having to go to the doctor because of flu by nearly 60%.
Every year, the Centers for Disease Control and Prevention (CDC) conduct studies to find out how well the flu vaccine protects against flu illness.
"This means that getting a flu vaccine this season reduced the risk of having to go to the doctor because of flu by nearly 60%," says Dr. Joseph Bresee, chief of the CDC's Epidemiology and Prevention Branch, commenting on this season's estimate.
The figure was announced at a recent meeting of the CDC's Advisory Committee for Immunization Practices (ACIP) in Atlanta, GA.
While flu vaccine effectiveness can vary, the CDC recommend an annual flu vaccine as the best way to prevent seasonal flu. The agency recommends the annual flu vaccine for everyone aged 6 months and older.
One of the highest levels of vaccine effectiveness on record
Since the CDC began estimating vaccine effectiveness in 2004-2005, records show the lowest level of effectiveness was for that season (10%), with the highest (60%) produced for the 2010-2011 season.
Last season (2014-2015), the vaccine was only 23% effective.
The seasonal flu vaccine is designed to protect against strains of flu viruses that researchers predict are likely to be most common during the coming flu season.
This season's preliminary 59% estimated vaccine effectiveness breaks down into more specific estimates for each of the following strains:
- 51% effective against the H1N1 viruses responsible for most flu illness this season
- 76% against all influenza B viruses
- 79% against the B/Yamagata lineage of B viruses.
The CDC note that there is not enough data yet to give estimates by age group, nor to estimate vaccine effectiveness against H3N2 viruses or B/Victoria lineage viruses.
CDC surveillance via FluSurv-NET shows that since the beginning of October 2015, there have been 1,147 laboratory-confirmed flu-related hospitalizations.
The highest rates of hospitalization this season are among people aged 65 and over, followed by children under 5 years of age, and then adults aged 50-64. During most seasons, children under 5 and adults 65 and over are the groups with the highest rates of hospitalization due to seasonal flu.
Vaccine designed to protect against infection and severity of flu illness
The seasonal flu vaccine aims to protect against infection (catching the virus) and illness; if you are vaccinated and become infected with the virus, you should experience a less severe reaction.
How well a flu vaccine works to prevent illness depends on two things: factors in the person being vaccinated (such as their age and health) and how well the vaccine matches against the viruses spreading in the community.
The CDC estimate for vaccine effectiveness represents the reduction in risk the vaccine provides. This is calculated from studies that measure laboratory-confirmed flu illness that results in a doctor's visit or urgent care visit.
Consequently, the CDC's vaccine effectiveness figure is not just a measure of how well the vaccine protects against catching the flu - it also takes into account the severity of illness if vaccinated people still become infected.
The viruses selected for inclusion in the vaccine are decided upon according to three factors: which flu viruses are circulating around the world, how they are spreading and how well current vaccine components protect against new strains.
Currently, 142 national influenza centers in 113 countries carry out year-round surveillance and study flu trends around the world. They send samples for analysis to five World Health Organization (WHO) collaborating centers, one of which being the CDC in Atlanta, GA.
Flu vaccine does not protect against other flu-like viruses
Once the decision about which strains to include in the vaccine is made, there is a time lag of many months while the new vaccine is developed and mass-produced. In the meantime, the situation can change and the strains predicted to thrive can end up not being the ones that actually dominate.
There are many viruses besides flu viruses that can result in flu-like illness that spread during the flu season - the seasonal flu vaccine does not protect against these.
Such non-flu viruses include, for example, the one that causes the common cold (rhinovirus) and respiratory syncytial virus (RSV) - the most common cause of severe respiratory illness in young children and a leading cause of death from respiratory illness in seniors.
Antiviral drugs - obtained by doctor's prescription - are an important second line of defense against the flu. The CDC also recommend that good hygiene habits - covering your cough and frequently washing your hands with soap, for example - can help stop flu and other respiratory illnesses from spreading.
Summing up the situation so far, Dr. Bresee says that flu activity this season seems to have started a bit later than usual and has been lower than expected compared with the previous three seasons. However, he notes that "activity is still on the upswing and expected to continue for several weeks," concluding:
"Flu causes serious illnesses and deaths every year. This season, CDC has received reports of hospitalizations and deaths in young, otherwise healthy people who were infected with influenza A H1N1, but not vaccinated."
Meanwhile, Medical News Today recently reported how a national poll finds that most American parents who do not have their children vaccinated against the flu do not rate the flu vaccine as highly as other childhood vaccines. The poll also reveals that parents find health care professionals talk differently about flu vaccine compared with other vaccines.