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Press Release

For Immediate Release: January 24, 2000
Contact: CDC Media Relations (404) 639-3286

Update: Flu Season 1999-2000


During the past 6-8 weeks, the proportion of patients seen by a sentinel network of physicians for influenza-like illness, the number of states and territories reporting "widespread" and "regional" influenza activity, and the numbers and types of circulating influenza viruses are similar to the previous two seasons (1997-98 and 1998-99). These findings suggest that this year's flu season has not been unusually severe. However, the reported deaths from pneumonia and influenza (P&I) deaths are higher this year. The causes of high P&I mortality levels are uncertain, but these figures should be interpreted with caution since they are preliminary and, in part, reflect changes made this year to the P&I reporting case definition. In addition, there is the possibility that other factors, such as other respiratory infections, could be contributing to the increased P&I mortality.

Background information on CDC's influenza surveillance systems

Each year, CDC actively monitors the flu season through several surveillance systems. These systems: 1) track the number (and percentage) of respiratory specimens testing positive for influenza virus and the numbers of influenza isolates identified by a group of >100 laboratories in the U.S.; 2) track the number of patient visits for influenza-like illness to a group of approximately 400 sentinel physicians located throughout the U.S.; 3) track state and territorial influenza activity levels as estimated and reported by the state or territorial epidemiologist; and 4) track the number of deaths in 122 cities attributed to pneumonia and influenza (P&I deaths). In addition, CDC receives reports about local outbreaks from state health departments and other sources.

During December through mid-January of the current 1999-2000 flu season, the percentages of respiratory specimens testing positive for influenza, the numbers and types of influenza viruses that were isolated and characterized, the numbers of patients visiting sentinel physicians for influenza-like illness, and the numbers of states and territories reporting widespread or regional influenza activity were similar to levels seen in the past two years. All three seasons have been predominated by influenza A(H3N2) Sydney-like viruses. Since P&I mortality levels typically lag behind virus and clinical illness activity markers by about 2 or 3 weeks, CDC 's earlier assessments of influenza activity levels were based on the levels of viral isolations and clinical illness activity.

During the week ending January 15, the percentage of P&I deaths in the 122 cities system reached 10.5%, which is higher than is usually seen. This level could indicate that this year's flu season is more severe than usual. However, before this can be concluded, some other important considerations must be taken into account.

  • First, the 122 cities mortality system is a rapid mortality monitoring system and its results are considered preliminary. The final assessment of an influenza season's severity is based on complete national death data, which usually are not available for analysis until 2 to 3 years after a calender year.
  • Second, a somewhat broader reporting case definition was implemented in the 122 cities mortality system for this flu season. This is an important change in the methods that could contribute to an increase in this season's P&I mortality estimates. Because the impact of the case definition change is uncertain, CDC has cautioned since the early part of the 1999-2000 season that the P&I figures must be interpreted with caution.
  • Third, as is true every year, levels of P&I mortality can be influenced by several other factors in addition to influenza. These factors include simultaneous circulation of other respiratory pathogens, such as respiratory syncytial virus. Higher levels of such pathogens could contribute to higher than usual levels of P&I mortality.
  • Fourth, it is important to note that 3 of the 4 surveillance systems did not indicate that this was an unusually severe year.

Can you put this year's P&I mortality level into context. How much higher than usual is this level of P&I mortality?

P&I mortality levels can vary substantially from season to season. During the previous three flu seasons (1996-97 through 1998-99), all of which were predominated by influenza A(H3N2) viruses, P&I mortality levels in the 122 cities system peaked at 9.1% in the 1996-97 season, 9.0% in the 1997-98 season, and 8.8% in the 1998-99 season.

What is CDC doing to determine whether this was a real increase in P&I mortality and if the season was more severe than usual?

Similar to what is done for every season, complete national mortality data will be analyzed when they are made available. This is usually 2-3 years after a calender year. Laboratory work on characterizing this years influenza viruses will continue. CDC will work with state and local health departments to investigate unusual outbreaks. Finally, CDC will be working with investigators to analyze data from managed care organizations to determine vaccine effectiveness estimates for this season. These results usually are not available until the late summer.

Why are both pneumonia and influenza deaths monitored? Why not influenza deaths alone?

Most deaths from influenza result from complications of this infection, including bacterial pneumonia. Following changes in pneumonia and influenza deaths is a common approach for monitoring influenza-related mortality.

The A Sydney virus strain has been circulating for three years. Is there anything unusual about this year's strain that is making it more virulent?

So far, no laboratory data have shown the Sydney viruses circulating this season to be substantially different from those that circulated in the previous 2 seasons. Nonetheless, laboratories at CDC work year-round to characterize circulating influenza viruses and this will continue.

Are people at high risk for complications from flu not getting vaccinated and might that explain the higher mortality rates?
Every year, large numbers of people who are at risk for serious complications from influenza -- either on the basis of age or because of underlying chronic medical conditions -- go unvaccinated. We do not know, whether larger numbers of high-risk people than usual went unvaccinated this year.

Is this year's vaccine working?

The match this year is very good between the circulating influenza viruses and the viruses in the flu vaccine. Otherwise, CDC does not have data at this time from studies that directly assess vaccine effectiveness. Results from vaccine effectiveness studies will be available toward the end of this summer.

If there is nothing unusual about the flu strain, could something else be causing the large number of deaths?

First, for this flu season, a somewhat broader reporting case definition was implemented in the 122 cities mortality system. This is an important change in the methods that could be inflating this season's P&I mortality estimates. Because the impact of the change in the case definition is uncertain, CDC has cautioned since the early part of the season that the P&I figures must be interpreted with caution. In addition, high levels of other respiratory pathogens, such as respiratory syncytial virus (RSV), could also contribute to higher levels of P&I mortality. RSV has been widespread this winter.

Should people still get a flu shot?

Any individual who is at high-risk for serious complications from influenza and who has not been vaccinated for the 1999-2000 season should still receive flu vaccine if vaccine is available locally. Because vaccine supply usually is limited and variable from area to area at this time of year, individuals desiring to be vaccinated should make inquiries to their health care providers about whether vaccine is available.

If influenza surveillance information lags by a few weeks, then how can people act on this information?

By far, the single most important step that an individual can take to prevent flu and its complications is to get vaccinated in the fall. Vaccination is particularly important for people who are at high risk for complications of flu and for those who can transmit influenza to such persons. A person should not use surveillance information to get vaccinated.

Although influenza surveillance information is useful for alerting physicians and helping to guide treatment decisions, the primary purposes of are influenza surveillance are to monitor changes in circulating influenza viruses, to monitor influenza activity levels and impact, and to provide warning about pandemic viruses.

Why do some people who get the flu vaccine still get the flu?

Every year, some people who get the flu vaccine will develop influenza-like illnesses. This can be due to various factors. First, even in the best situation, the flu vaccine is not 100% effective in preventing illness due to influenza (see, "Is this year's vaccine working?"). However, even when it does not prevent illness, the flu vaccine can still prevent severe illness and death due to influenza. Second, symptoms similar to symptoms of influenza can be caused by other viruses or bacteria. The flu vaccine will not protect a person from illness caused by agents other than influenza viruses. The only way to be sure that an illness is flu is to test for the influenza virus.

Does CDC recommend a second or booster vaccination for influenza?

A second vaccination (at least 1 month later) is recommended only for previously unvaccinated children <9 years of age. Otherwise a booster vaccination is not recommended.

How many people have died, so far, during this flu season?

The average number of deaths each year from influenza in the U.S. is approximately 20,000. The final estimated numbers of P&I deaths are based upon complete national death data. The data for this year will not be available for about 2-3 years.

Follow this link for additional Questions and Answers about the 1999-2000 Flu season (


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