For years Newtown Health District Director Donna Culbert has been recommending that residents be proactive and always get their flu shots to create the most robust protection against contracting the annual strains of influenza that circulate throughout much of the winter season.

But this year, Ms Culbert is being proactive by clarifying information about a so-called “shift” or “drift” in one of the flu strains current vaccines were designed to attack, to be sure residents do not hear misinformation and opt out of getting their annual shot because they believe it is no longer effective.

Flu season has begun, and it is true that the vaccine in current circulation does not protect as well against the dominant strain seen most commonly so far this year. It is also true that this strain tends to cause more deaths and hospitalizations, especially in the elderly.

Every year, health officials work hard to predict the major strains of influenza they believe will circulate to various populations and regions, and manufacturers design vaccines to combat those particular strains. Ms Culbert said depending on the year, or the vaccine, each shot typically protects recipients from three or four of the flu strains experts believe will be most prevalent.

This year, however, she said one of the viruses shifted, making that one of the antibodies in the vaccine less effective than normal - but it does not render the antibody for that strain, or the vaccine itself, ineffective.

“My understanding is the H3N2 strain has shifted, so that part of the vaccine’s effectiveness is somewhat reduced,” the local health official told The Bee December 10. “But by no means should this indicate to residents to not get their flu shot, especially those that are at the highest risk like the very young, elderly, and those who are medically fragile or whose immune systems may be compromised.”

Not As Effective

CDC officials think the vaccine should provide some protection and still are urging people to get vaccinated. But it probably will not be as good as if the vaccine strain was a match.

Flu vaccine effectiveness tends to vary from year to year. Last winter, flu vaccine was 50 to 55 percent effective overall, which experts consider relatively good.

But Dr Tom Frieden, director of the Centers for Disease Control and Prevention, is concerned that the United States may see a flu season that is more severe than most.

Two strains currently circulating are subtypes of the A(H3N2) virus, but the newer one is not fully covered by this year’s influenza vaccination. About half of A(H3N2) cases reported to the Centers for Disease Control and Prevention, or CDC, are associated with the new strain.

While the flu is not occurring with great frequency yet in Connecticut, Ms Culbert said at least one regional hospital was reporting an increase in respiratory infections that are not flu related.

She said, however, that anyone experiencing the combination of respiratory infection and the onset of weakness that can signal the flu should react quickly and contact their physician, even if they already received a flu shot this season.

“Anyone feeling flu-like symptoms, especially those already medically compromised, should contact their physician immediately to determine whether they should begin taking an antiviral medication,” Ms Culbert said. “If they are able to begin a course of antiviral medicine within 24 to 48 hours of the onset of influenza, it could help reduce the intensity or the duration of the flu.

“But it won’t prevent it,” she added.

Antigenic ‘Drift’

According to the CDC, influenza viruses are constantly changing, and they can change in two different ways.

One way they change is called “antigenic drift.” These are small changes in the genes of influenza viruses that happen continually over time as the virus replicates.

These small genetic changes usually produce viruses that are pretty closely related to one another, which can be illustrated by their location close together on a phylogenetic tree. Viruses that are closely related to each other usually share the same antigenic properties and an immune system exposed to an similar virus will usually recognize it and respond. (This is sometimes called cross-protection.)

But these small genetic changes can accumulate over time and result in viruses that are antigenically different (further away on the phylogenetic tree). When this happens, the body’s immune system may not recognize those viruses.

This process works as follows: a person infected with a particular flu virus develops antibodies against that virus. As antigenic changes accumulate, the antibodies created against the older viruses no longer recognize the “newer” virus, and the person can get sick again.

Genetic changes that result in a virus with different antigenic properties is the main reason why people can get the flu more than one time. This is also why the flu vaccine composition must be reviewed each year, and updated as needed to keep up with evolving viruses.

Antigenic ‘Shift’

The other type of change is called “antigenic shift.” Antigenic shift is an abrupt, major change in the influenza A viruses, resulting in new proteins in influenza viruses that infect humans. Shift results in a new influenza A subtype or a virus with a protein combinations that have emerged from an animal population that is so different from the same subtype in humans that most people do not have immunity to the new (e.g., novel) virus.

Such a “shift” occurred in the spring of 2009, when an H1N1 virus with a new combination of genes emerged to infect people and quickly spread, causing a pandemic. When shift happens, most people have little or no protection against the new virus.

While influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.

Associated Press content was used in this report.