Updated May 26, 2015.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
New Flu Virus Strains
Flu viruses change all of the time. That's why we need to get a flu shot every year.
Some of these changes are minor, but there are sometimes big changes that create completely new flu virus strains. It is these new flu virus strains that experts worry about because they are the ones that can trigger a flu pandemic.
Other things to know about new flu virus strains include that:
- people, especially younger children, often don't have any immunity to "novel" or new animal flu virus strains
- swine flu virus strains that can get people sick are called variant flu viruses
- some new flu virus strains have genetic changes that can make it easier for them to spread from animals to people or to cause more serious disease
Minor changes in flu virus strains are caused by the process of antigenic drift. Mutations might cause small changes in the flu virus so that our antibodies don't recognize it and we aren't fully protected against infection.
More major changes are caused by antigenic shift. This is what triggered the 2009 H1N1 pandemic. Instead of a simple mutation or change in a single flu strain, an antigenic shift results from a larger reassortment of genetic material from different flu strains, usually between animal and human strains. So through an antigenic shift, a flu virus strain that might have only been able to infect birds or pigs, might then gain the ability to spread in humans.
2014-1015 Flu Season
The flu virus strains that experts predicted would be the most common during the 2014-1015 flu season and which were included in this season's flu vaccine included:
- an A/California/7/2009 (H1N1)pdm09-like virus
- an A/Texas/50/2012 (H3N2)-like virus
- a B/Massachusetts/2/2012-like virus
- a B/Brisbane/60/2008-like virus (quadrivalent flu vaccines only)
The H1N1 flu virus that caused a pandemic has now become just another seasonal flu virus.
These flu virus strains in the 2014-15 flu vaccines are essentially unchanged from the 2013-14 flu season. That doesn't mean you can skip your vaccine though if you had a flu shot last year. Immunity to flu viruses drop over time, even with natural infections, and so it is still recommended that everyone get a flu vaccine each year, even when the flu vaccines don't change much, or at all.
Unfortunately, a drifted H3N2 flu strain made the flu vaccine less effective this year, in what ended up being a severe flu season.
Next year's 2015-16 flu vaccine strains will include:
- an A/California/7/2009 (H1N1)pdm09-like virus
- an A/Switzerland/9715293/2013 (H3N2)-like virus
- and a B/Phuket/3073/2013-like (B/Yamagata lineage) virus
- quadrivalent vaccines will also include a B/Brisbane/60/2008-like (B/Victoria lineage) virus
So up to two components of next year's flu vaccine will change.
Dog Flu
Although many of us have gotten used to hearing about bird flu and swine flu, dog flu is a new thing.
Hearing about the 2015 dog flu (canine influenza) outbreak in Chicago probably both surprised and worried many people. Fortunately, dog flu isn't contagious to people. This H3N2 virus that originated in horses can easily spread between dogs though, which makes it important to try to control and prevent new cases.
HPAI H5
Since December 2014, highly pathogenic avian influenza (HPAI) H5 virus strains have been discovered in the United States in wild, migratory birds (Canada Goose, Mallards, Snowy Owl, etc.) and domestic poultry, including chickens, turkeys and ducks.
Millions of sick birds in at least 20 states have led to recommendations from the CDC that people:
- avoid wild birds and observe them only from a distance
- avoid contact with domestic birds (poultry) that appear ill or have died
- avoid contact with surfaces that appear to be contaminated with feces from wild or domestic birds.
The health risk to people from this bird flu is thought to be low at this time though.
There is a concern that the bird flu outbreak could lead to egg shortages, higher prices for eggs, and higher prices for turkey.
H3N2 Variant Infections
A variant H3N2 (H3N2v) virus got 12 people sick in 2011. This is a flu virus strain that typically infects pigs, but was able to get people sick - mostly people who had prolonged contact to infected pigs.
This is an ongoing outbreak. In 2012, the case count for H3N2v infections increased to 309 in 12 states.
Although we also saw some cases since then, as in previous years, there is limited spread from one person to another. That's good news for all of us, but certain people still need to be careful, especially those at high risk for flu complications. The CDC recommends that they avoid swine barns and pigs at agricultural fairs.
To avoid getting H3N2v from a pig, it is also important that everyone:
- wash their hands properly after any exposure to pigs
- avoid taking food and drinks around pigs
- avoid taking your child's stroller, toys, baby bottle, or pacifier, etc., around pigs
- stay away from any pig that is sick
Keep in mind that you can't always tell if a pig has H3N2v. As with human infections, some pigs can be infected with the virus and have only mild symptoms or no symptoms at all and can still be contagious to others.
And because flu virus strains can mix together and shift to new strains, you should avoid pigs if you are sick with the flu.
There were only 19 cases of H3N2v in the US in 2013 and 4 cases in 2014.
H7N9 Bird Flu Outbreak
The 2013 H7N9 outbreak in China had a lot of people concerned, as there were 44 deaths among just 132 cases in 2013. There have also been a few cases in travelers returning from China, including people who got sick in Canada and Malaysia.
Fortunately, this type of bird flu was never actually able to spread from one person to another. Instead, it is thought that people got sick from exposure to infected poultry, especially at live bird markets.
The early 2013 outbreak seemed to be over, but new cases seem to surge each year. There have now been at least 571 cases of H7N9 infections, mostly in China, and at least 212 deaths.
H5N1
In addition to H7N9, there is another strain of bird flu that has been around much longer - highly pathogenic avian influenza H5N1.
First discovered in 2003, there have been at least 664 cases of HPAI H5N1 in 15 countries and at least 391 deaths.
Most of the cases are in Asia and northeast Africa, especially Egypt and Indonesia. If you will be visiting one of these areas, the CDC does recommend that you "avoid visiting poultry farms, bird markets and other places where live poultry are raised, kept, or sold."
Like H7N9, this type of bird flu does not seem to spread between people.
H9N2
Two new cases of avian influenza A (H9N2) were also confirmed in 2013, both in China.
This type of bird flu tends to cause mild symptoms, so is not thought to be a big threat right now.
H10N8
A new strain of new cases of avian influenza to infected humans, A (H10N8), has now been confirmed to have infected at least two people in China since January 2014.
In one of the cases, a 55-year-old woman who had been to an agricultural market developed severe pneumonia and was in critical condition. Another person with an H10N8 infection died in late 2013.
What You Need To Know About New Flu Strains
It can be a bit scary thinking that every new flu strain could trigger a new flu pandemic and a large number of deaths from flu. Fortunately, these types of pandemics are rare.
Other things to know about new flu virus strains include that:
- Antiviral flu drugs, including Tamiflu (oseltamivir) and Relenza (zanamivir) can usually be used to treat new flu strains, even when a vaccine isn't yet available.
 - New flu virus strains continue to change, which can mean that the HPAI H5N1 virus could one day develop the ability to spread more easily from one person to another.
 - Both the H and N components of H7N9 are new strains that had not previously gotten people sick. In contrast, only the H component of H5N1 is new among people.
 - There have been limited cases of H7 flu in North America in the past, including H7N3 in Canada in 2004 (2 human cases), H7N2 in New York in 2003 (one human case), and H7N2 in Virginia in 2002 (one case). Unlike H5N1, these were were all low pathogenic avian influenza virus strains.
 - The World Health Organization has a global influenza surveillance and monitoring program to help find and identify new strains of flu.
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Most importantly, a universal flu vaccine that could protect against all strains of the flu will hopefully one day protect us all from any new flu virus strains.
Until then, it is important that we continue to watch for and monitor these new flu virus strains, work on the production of new antiviral medications, new vaccines, and methods to decrease the spread of these flu viruses from animals to people. And be sure to tell your doctor if you develop flu symptoms and have recently visited an area where these new flu virus strains are known to be getting people sick.
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Sources:
CDC. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP) — United States, 2014–15 Influenza Season. MMWR. August 15, 2014 / 63(32);691-697.
CDC. Avian Influenza A (H7N9) Virus. http://www.cdc.gov/flu/avianflu/h7n9-virus.htm. Accessed July 2013.
WHO. Monthly Risk Assessment Summary. http://www.who.int/influenza/human_animal_interface/HAI_Risk_Assessment/en/index.html Accessed February 2014.
CDC. Influenza A (H3N2) Variant Virus. http://www.cdc.gov/flu/swineflu/h3n2v-cases.htm. Accessed July 2013.
Diana L. Adapting global influenza management strategies to address emerging viruses. American Journal of Physiology - Lung Cellular and Molecular Physiology. 2013 Jul;305(2):L108-17.
Sasiesekharan, Ram. Structural Determinants for Naturally Evolving H5N1 Hemagglutinin to Switch Its Receptor Specificity. Cell, Volume 153, Issue 7, 1475-1485, 06 June 2013.