Synopsis:

· The 2008-09 influenza season ran from September 28, 2008, to May 23, 2009.

· The peak level of activity was observed between February 15 and March 7, 2009.

· About 5,800 visits for ILI were reported by sentinel providers from over 127,000 visits.

· About 5,400 lab-confirmed cases were reported by collaborating laboratories from over 40,000 tests performed.

· About 500 hospitalizations for influenza were reported by participating hospitals.

· Type B influenza and Type A (H1N1)-Seasonal influenza were the predominant strains this flu season. Type A (H1N1)-”Swine” influenza appeared toward the end of the regular flu season and, as of this report, accounted for about 10% of all isolates reported by the DHMH Laboratories Administration.

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U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet)

During the 2008-09 influenza season, 20 sentinel providers actively participated in reporting the number of visits for ILI at their practices. A total of 5,766 visits for ILI were reported during the 2008-09 influenza season from a total of 126,967 total visits to sentinel providers.

The 5-24 age group accounted for 37% of ILI visits, followed by the 25-64 age group with 33% of ILI visits, then the 0-4 age group with 19% of  ILI visits, and the over 65 age group with 11% of ILI visits.

The median percent of ILI visits was above the 3.6% baseline for six weeks between February 1 and March 14, 2009.

Reports From Sentinel Laboratories

A total of 36 laboratories actively participated in influenza surveillance during the 2008-09 season. These laboratories reported that 40,177 rapid tests were performed, including 5,118 (13%) positive results.

Type A influenza was detected in 18% of these samples; type B influenza in 12%. About 70% of the samples were not typed by the rapid influenza tests. The peak number of tests performed and positives detected was reported between February 15 and March 7, 2009. The peak percent positive of rapid tests was 22% on the week ending March 7, 2009.

For purposes of influenza surveillance, positive rapid influenza tests are counted as lab-confirmed cases only after DHMH Laboratories report a positive influenza test by PCR and/or viral culture. This is because the positive predictive value (accuracy) of the rapid tests vary with the prevalence of the virus in the community. That is, when there is a low level of virus, more positive rapid tests will be false-positives. When there is a high level of virus, more positive rapid tests will be true-positives.

Reports From DHMH Laboratories Administration

A total of 1,323 PCR and/or viral culture tests were reported performed by the DHMH Laboratories Administration. Of these, 398 (30%) were positive. Among the positives, 167 (42%) were type B and 166 (41%) were type A(H1N1)-Seasonal. Type A(H1N1)- Swine Origin isolates began to appear on the week ending May 2, 2009.

Maryland Resident Influenza Tracking Survey

A total of 695 Maryland Residents signed up to participate between October 5, 2008, and May 23, 2009. Participants were from every county in Maryland and Baltimore City. The average age of participants was 45 years (median  45 years), with some participants as young as 1 year of age and as old as 81 years of age. Over 45% of participants responded every week. A total of 182 (26%) participants reported flu-like symptoms at one or more points in the flu season. Collectively, those reporting flu-like symptoms reported missing 379 days of regular daily activities, like going to school or work, and 59 (32%) of those with flu-like symptoms reported seeking medical care.

The weekly percent of participants with flu-like illness followed the ILINet percent ILI closely, rising and falling in a near-parallel manner. The peak percent of MRITS participants reporting flu-like illness occurred during the week ending February 21, 2009. Similarly, the peak percent of ILI visits to sentinel providers occurred a week later, during the week ending February 28, 2009. Proportions of ILI among MRITS participants declined as the season progressed and then increased again with the onset of the H1N1 “Swine” flu.

Emerging Infections Program (EIP) Reports

During the 2008-09 influenza season, a total of 499 hospitalizations were reported to by 21 participating hospitals. Of these, 241 (48%) were children under the age of 18 and 258 (52%) were adults.

The peak number of reported hospitalizations occurred during the week of March 7 to 13, 2009, with 79 hospitalizations.

Last season, 715 hospitalizations were reported by 21 hospitals. Of those, 199 (28%) were children under the age of 18 and 516 (72%) were adults. The peak during that season occurred about four weeks earlier than it did this flu season, when 100 hospitalizations were reported in one week.

Division of Outbreak Investigation Reports

During the 2008-09 influenza season, a total of 35 outbreaks of respiratory disease were reported to the Division of Outbreak Investigation. Of these, 15 (43%) were outbreaks of pneumonia, 9 (26%) were outbreaks of influenza, 8 (23%) were outbreaks of influenza-like illness, and 3 (9%) were outbreaks of other respiratory conditions.

It must be noted that the six influenza outbreaks observed at the end of the season are associated with Type A(H1N1) - Swine Origin influenza, and were likely the result of enhanced surveillance for these cases in institutional settings such as schools.

Comparatively, during the 2007-08 influenza season, A total of 60 respiratory outbreaks in were reported. Of those, 29 (48%) were confirmed by lab testing as being caused by influenza. The other 31 (52%) were classified as ILI outbreaks only if influenza testing was unavailable. Pneumonia outbreaks were not included in the analysis of the 2007-08 influenza season, but there were several reported.

ESSENCE ILI Reports

Surveillance of chief complaints from persons visiting emergency departments at hospitals participating in ESSENCE showed a peak in the percent of persons complaining of flu-like illness during the week ending February 28, 2009. That week, 4% of over 15,400 visits to participating Emergency Departments were for ILI.

While the total number of visits and visits of ILI increased toward the end of the season, this was due to additional hospitals being included in the system. Also, the increase in the percentage of complaints of ILI toward the end of the season may be attributed to public concern over TypeA(H1N1) - Swine Origin influenza.

To read more about surveillance conducted by the Office of Preparedness and Response, please visit their web page at http://bioterrorism.dhmh.state.md.us.