Key indicators that track flu activity remain high and, after falling during the first two weeks of the year, increased over the last two weeks.

Key Points

• Outpatient ILI and laboratory data remain elevated and increased again this week. Nationally and in some regions the proportion of influenza A(H1N1)pdm09 viruses compared to influenza B viruses is increasing.
• Overall, hospitalization rates remain similar to what has been seen at this time during recent seasons, but rates among children and young adults are higher at this time than in recent seasons.
• Pneumonia and influenza mortality has been low, but 68 influenza-associated deaths in children have been reported so far this season.
• CDC estimates that so far this season there have been at least 19 million flu illnesses, 180,000 hospitalizations and 10,000 deaths from flu.
• Flu vaccine effectiveness estimates are not available yet this season, but vaccination is always the best way to prevent flu and its potentially serious complications.
• Antiviral medications are an important adjunct to flu vaccine in the control of influenza.
  
  
  
  Almost all (>99%) of the influenza viruses tested this season are susceptible to the four FDA-approved influenza antiviral medications recommended for use in the U.S. this season.

Influenza (flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness. Serious outcomes of flu infection can result in hospitalization or death. Some people, such as older people, young children, and people with certain health conditions, are at high risk of serious flu complications.  There are two main types of influenza (flu) virus: Types A and B. The influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year.

The best way to prevent flu is by getting vaccinated each year.

What's new this flu season?

A few things are new this season:

• Flu vaccines are updated to better match viruses expected to be circulating in the United States.
  • The A(H1N1)pdm09 vaccine component was updated from an A/Michigan/45/2015 (H1N1)pdm09-like virus to an A/Brisbane/02/2018 (H1N1)pdm09-like virus.
  • The A(H3N2) vaccine component was updated from an A/Singapore/INFIMH-16-0019/2016 A(H3N2)-like virus to an A/Kansas/14/2017 (H3N2)-like virus.
  • Both B/Victoria and B/Yamagata virus components from the 2018-2019 flu vaccine remain the same for the 2019-2020 flu vaccine.
• All regular-dose flu shots will be quadrivalent. (No trivalent regular-dose flu shots will be available this season.)
• All recombinant vaccine will be quadrivalent. (No trivalent recombinant vaccine will be available this season.)
• All four of the vaccine viruses used to produce cell-grown flu vaccine will have been grown in cells, not eggs.
• In January 2019, the U.S. Food and Drug Administration (FDA) approved a change in dose volume for Fluzone Quadrivalent, a quadrivalent inactivated influenza vaccine.
  • The change in dose volume affects children 6 through 35 months of age.
  • Previously, children in this age group were recommended to receive 0.25 milliliters of this vaccine per dose.
  • Children 6 through 35 months of age may now receive either 0.25 milliliters or 0.5 milliliters per dose.
  • There is no preference for one or the other dose volume for this age group. All persons 36 months (or 3 years) of age and older should receive 0.5 milliliters per dose.
• In October 2018, FDA approved an expanded age indication for Afluria Quadrivalent, a quadrivalent inactivated influenza vaccine. Afluria Quadrivalent is now licensed for children 6 months of age and older. Children 6 months through 35 months of age should receive 0.25 milliliters for each dose. All persons 36 months (or 3 years) of age and older should receive 0.5 milliliters for each dose.
• Baloxavir marboxil (trade name Xofluza?) is a new flu single-dose antiviral drug approved October 24, 2018 by FDA. Baloxavir is currently approved by the FDA for treatment of acute uncomplicated influenza within 2 days of illness onset in people 12 years and older who are otherwise healthy, or at high risk of developing influenza-related complications. More information is available here: Influenza Antiviral Drug Baloxavir Marboxil.

• For the 2019-2020 flu season, CDC is offering additional guidance on timing of flu vaccination for both adults and children.
  • Like last season, CDC and ACIP recommend that vaccination be offered by the end of October.
    • For this season, CDC and ACIP provided additional information on what might be considered vaccinating too early. "Vaccinating early - for example, in July or August -may lead to reduced protection against influenza later in the season, particularly among older adults."
  • Children 6 months through 8 years of age who need 2 doses should receive their first dose as soon as possible after vaccine becomes available to allow the second dose (which must be administered at least 4 weeks later) to be received by the end of October.

For more information on 2019-2020 influenza vaccine recommendations: Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices -- United States, 2019-20 Influenza Season

What flu vaccines are recommended this season?

For the 2019-2020 flu season, providers may choose to administer any licensed, age-appropriate flu vaccine (IIV, RIV4, or LAIV4).

Options this season include:

• Standard dose flu shots. Like all inactivated influenza vaccines, these are given into the muscle. They are usually given with a needle, but one (Afluria Quadrivalent) can be given to some people (those 18 to 64 years old) with a jet injector.
• High-dose shots for people 65 years and older.
• Shots made with adjuvant for people 65 years and older.
• Shots made with virus grown in cell culture. No eggs are involved in the production of this vaccine.
• Shots made using a vaccine production technology (recombinant vaccine) that does not require the use of flu virus.
• Live attenuated influenza vaccine (LAIV). - A vaccine made with attenuated live virus that is given by nasal spray vaccine.

There is a table showing all flu vaccines that are FDA-approved for use in the United States during the 2019-2020 season.

What viruses will the 2019-2020 flu vaccines protect against?

There are many different flu viruses and they are constantly changing. The composition of U.S. flu vaccines is reviewed annually and updated as needed to match circulating flu viruses. Flu vaccines protect against the three or four viruses (depending on the vaccine) that research suggests will be most common. For 2019-2020, trivalent (three-component) vaccines are recommended to contain:

• A/Brisbane/02/2018 (H1N1)pdm09-like virus (updated)
• A/Kansas/14/2017 (H3N2)-like virus (updated)
• B/Colorado/06/2017-like (Victoria lineage) virus

Quadrivalent (four-component) vaccines, which protect against a second lineage of B viruses, are recommended to contain:

• the three recommended viruses above, plus B/Phuket/3073/2013-like (Yamagata lineage) virus.

FDA's Vaccines and Related Biological Products Advisory Committee (VRBPAC) chooses the flu vaccine viruses for the United States. For 2019-2020 vaccines, they selected the H1N1 and B components on March 6, 2019.external icon Selection of the H3N2 component was delayed until March 22, 2019.

How well-matched are 2019-2020 vaccine viruses to circulating flu viruses? How well is flu vaccine protecting against illness?

It's understandable that people want to know how well flu vaccines are working or are expected to work this season. CDC does not have flu vaccine effectiveness estimates for this season yet because it is still early in the season  and these estimates are based on epidemiologic studies comparing illness among vaccinated versus unvaccinated people. That data will be available later.

In the meantime, laboratory data can provide some insight into how well vaccines might work. The most helpful data for this is the antigenic characterization data, which are updated weekly in FluView. Limited antigenic data on recently circulating viruses are available at this point in the season, and the data suggest similarity of the circulating influenza A(H1N1)pdm09 and B/Yamagata viruses tested so far to the vaccine viruses. However, the influenza B/Victoria and A(H3N2) viruses that have been tested show some reduced similarity to the vaccine viruses. Again, at this point in the flu season, the antigenic data are limited and can only give early insights into how well vaccines might work. More information about how CDC antigenically characterizes flu viruses is available, and more complete antigenic data will be available in the coming weeks.

During past seasons when vaccine viruses were antigenically "like" most circulating viruses, vaccine effectiveness in the range of 40% to 60% has been observed. This means that people who get vaccinated may still get sick, but they are about half as likely to get sick as someone who was not vaccinated. Another important thing to remember is that vaccination may make illness less severe in people who get vaccinated and still get sick. In general people who get vaccinated are better off than people who do not get vaccinated. It's important to remember though, that things can change very quickly with flu and we could still see circulation of flu viruses with significant antigenic drift this season.