Weekly Report: Influenza Summary Update

2008-2009 Influenza Season Week 25 ending June 27, 2009

All data are preliminary and may change as more reports are received.

On June 11, the World Health Organization raised the pandemic alert level from Phase 5 to Phase 6 indicating that an influenza pandemic is underway. The novel influenza A (H1N1) virus now will be referred to as “pandemic H1N1 influenza virus.”

Synopsis:

During week 25 (June 21-27, 2009), influenza activity decreased in the United States, however, there were still higher levels of influenza-like illness than is normal for this time of year.

• Two thousand one hundred seventy (30.7%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories and reported to CDC/Influenza Division were positive for influenza.
• Over 98% of all subtyped influenza A viruses being reported to CDC were pandemic influenza A (H1N1) viruses.
• The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.
• Eight influenza-associated pediatric deaths were reported and seven of the eight deaths were associated with pandemic influenza A (H1N1) virus infection.
• The proportion of outpatient visits for influenza-like illness (ILI) was below the national baseline. Two of the 10 surveillance regions reported ILI above their region-specific baseline.
• Ten states reported geographically widespread influenza activity, 11 states reported regional influenza activity, 15 states reported local influenza activity, Puerto Rico and 12 states reported sporadic influenza activity, and the District of Columbia and two states did not report.

National and Regional Summary of Select Surveillance Components

HHS Surveillance Regions*	Data for current week			Data cumulative for the season
	Out-patient ILI†	% positive for flu‡	Number of jurisdictions reporting regional or widespread activity§	A (H1)	A (H3)	Pandemic A (H1N1)	A (unable to sub-type)¥	A (Sub-typing not performed)	B	Pediatric Deaths
Nation	Normal	30.7 %	21 of 52	7,883	2,334	19,321	734	15,544	10,552	84
Region I	Normal	30.5 %	4 of 6	521	151	1,684	13	1,591	800	3
Region II	Elevated	38.4 %	1 of 3	277	140	853	20	2,125	711	11
Region III	Normal	55.0 %	5 of 6	1,308	218	2,956	7	812	1,360	9
Region IV	Normal	30.8 %	4 of 8	829	129	683	46	2,199	1,217	6
Region V	Normal	52.7 %	2 of 6	1,650	195	7,605	166	828	1,409	15
Region VI	Normal	30.6 %	1 of 5	797	266	2,103	5	4,155	2,628	15
Region VII	Normal	16.8 %	1 of 4	522	73	330	184	480	535	0
Region VIII	Normal	19.8 %	0 of 6	530	217	988	57	1,549	499	7
Region IX	Normal	17.3 %	3 of 4	1,061	632	844	33	1,339	702	16
Region X	Elevated	23.3 %	0 of 4	388	313	1,275	203	466	691	2

* HHS regions (Region I: CT, ME, MA, NH, RI, VT; Region II: NJ, NY, Puerto Rico, US Virgin Islands; Region III: DE, DC, MD, PA, VA, WV; Region IV: AL, FL, GA, KY, MS, NC, SC, TN; Region V: IL, IN, MI, MN, OH, WI; Region VI: AR, LA, NM, OK, TX; Region VII: IA, KS, MO, NE; Region VIII: CO, MT, ND, SD, UT, WY; Region IX: AZ, CA, Guam, HI, NV; and Region X: AK, ID, OR, WA)
† Elevated means the % of visits for ILI is at or above the national or region-specific baseline
‡ National data are for current week; regional data are for the most recent three weeks
§ Includes all 50 states, the District of Columbia, and Puerto Rico
¥ The majority of influenza A viruses that cannot be sub-typed as seasonal influenza viruses are pandemic A (H1N1) influenza viruses upon further testing

U.S. Virologic Surveillance:

WHO and NREVSS collaborating laboratories located in all 50 states and Washington D.C. report to CDC the number of respiratory specimens tested for influenza.

During the 2008-09 season, influenza A (H1), A (H3), and B viruses have co-circulated in the United States. On April 15 and 17, 2009, CDC confirmed the first two cases of pandemic influenza A (H1N1) virus in the United States. As of July 2, 2009, 33,902 confirmed and probable infections with pandemic influenza A (H1N1) virus and 170 deaths (37 deaths in individuals less than 25 years, 133 deaths in adults 25 years of age older, and five deaths with unknown age) have been identified by CDC and state and local public health departments. Reporting of pandemic influenza A (H1N1) viruses by U.S. WHO collaborating laboratories began during week 17 (week ending May 2, 2009). The results of tests performed during the current week are summarized in the table below.

	Week 25
No. of specimens tested	7,072
No. of positive specimens (%)	2,170 (30.7%)
Positive specimens by type/subtype
Influenza A	2,165 (99.8%)
A (pandemic H1N1)	1,430 (66.1%)
	636 (29.4%)
A (unable to subtype)	73 (3.4%)
A (H3)	16 (0.7%)
A (H1)	10 (0.5%)
Influenza B	5 (0.2%)

During week 25, seasonal influenza A (H1), A (H3), and B viruses co-circulated at low levels with pandemic influenza A (H1N1) viruses. Over 98% of all subtyped influenza A viruses being reported to CDC this week were pandemic influenza A (H1N1) viruses.

The increase in the percentage of specimens testing positive for influenza by WHO and NREVSS collaborating laboratories may be due in part to changes in testing practices by health care providers, triaging of specimens by public health laboratories, an increase in the number of specimens collected from outbreaks, and other factors.

View WHO-NREVSS Regional Bar Charts| View Chart Data | View Full Screen

Antigenic Characterization:

CDC has antigenically characterized 1,729 seasonal human influenza viruses [995 influenza A (H1), 171 influenza A (H3) and 563 influenza B viruses] collected by U.S. laboratories since October 1, 2008, and 144 pandemic influenza A (H1N1) viruses.

All 995 influenza seasonal A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). All 171 influenza A (H3N2) viruses are related to the A (H3N2) vaccine component (A/Brisbane/10/2007).

All 144 pandemic influenza A (H1N1) viruses are related to the A/California/07/2009 (H1N1) reference virus selected by WHO as a potential candidate for pandemic influenza A (H1N1) vaccine.

Influenza B viruses currently circulating can be divided into two distinct lineages represented by the B/Yamagata/16/88 and B/Victoria/02/87 viruses. Sixty-seven influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 496 viruses belong to the B/Victoria lineage and are not related to the vaccine strain.

Data on antigenic characterization should be interpreted with caution given that antigenic characterization data is based on hemagglutination inhibition (HI) testing using a panel of reference ferret antisera and results may not correlate with clinical protection against circulating viruses provided by influenza vaccination.

Annual influenza vaccination is expected to provide the best protection against those virus strains that are related to the vaccine strains, but limited to no protection may be expected when the vaccine and circulating virus strains are so different as to be from different lineages, as is seen with the two lineages of influenza B viruses. Antigenic characterization of pandemic influenza A (H1N1) viruses indicates that these viruses are antigenically and genetically unrelated to seasonal influenza A (H1N1) viruses, suggesting that little to no protection would be expected from vaccination with seasonal influenza vaccine.

Antiviral Resistance:

Since October 1, 2008, 1,039 seasonal influenza A (H1N1), 191 influenza A (H3N2), and 571 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). One thousand forty-four seasonal influenza A (H1N1) and 204 influenza A (H3N2) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). Two hundred two pandemic influenza A (H1N1) viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). One hundred seventy-seven pandemic influenza A (H1N1) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). The results of antiviral resistance testing performed on these viruses are summarized in the table below.

	Isolates tested (n)	Resistant Viruses, Number (%)		Isolates tested (n)	Resistant Viruses, Number (%)
		Oseltamivir	Zanamivir		Adamantanes
Seasonal Influenza A (H1N1)	1,039	1,034 (99.5%)	0 (0)	1,044	6 (0.6%)
Influenza A (H3N2)	191	0 (0)	0 (0)	204	204 (100%)
Influenza B	571	0 (0)	0 (0)	N/A*	N/A*
Pandemic Influenza A (H1N1)	202	0 (0)	0 (0)	177	177 (100%)

        ^{*The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.}

The pandemic influenza A (H1N1) virus is susceptible to both neuraminidase inhibitor antiviral medications zanamivir and oseltamivir. It is resistant to the adamantane antiviral medications, amantadine and rimantadine. Antiviral treatment with either oseltamivir or zanamivir is recommended for all patients with confirmed, probable or suspected cases of pandemic influenza A (H1N1) virus infection who are hospitalized or who are at higher risk for seasonal influenza complications. Additional information on antiviral recommendations for treatment and chemoprophylaxis of pandemic influenza A (H1N1) infection is available at http://www.cdc.gov/h1n1flu/recommendations.htm

Three seasonal influenza A (H1N1) viruses collected between February 8 and May 11, 2009 were found to be resistant to both oseltamivir and the adamantanes (amantadine and rimantadine). All influenza A (H1N1) viruses tested retain their sensitivity to zanamivir. The three dually resistant viruses represent less than 0.5% of all seasonal influenza A (H1N1) viruses tested during the 2008-09 influenza season, and as a result, no changes to the influenza antiviral treatment or prophylaxis recommendations will be made at this time. CDC will continue to monitor trends in antiviral resistance over the summer and throughout the upcoming 2009-10 influenza season.

Pneumonia and Influenza (P&I) Mortality Surveillance

During week 25, 6.5% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage is below the epidemic threshold of 6.7% for week 25.

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Influenza-Associated Pediatric Mortality

Eight influenza-associated pediatric deaths were reported to CDC during week 25 (Arizona, New Jersey [2], Oregon, Rhode Island, Texas, Utah, and Wisconsin). Seven of these deaths were associated with pandemic influenza A (H1N1) virus infection and one death was due to influenza A virus (subtyping not performed). The deaths reported this week occurred during weeks 22-25 (the weeks ending June 6-27, 2009). Since September 28, 2008, CDC has received 84 reports of influenza-associated pediatric deaths that occurred during the current influenza season, 17 of which were due to pandemic influenza A (H1N1) virus infections.

Of the 37 children who had specimens collected for bacterial culture from normally sterile sites, 14 (37.8%) were positive; Staphylococcus aureus was identified in nine (64.3%) of the 14 children. Four of the S. aureus isolates were sensitive to methicillin and five were methicillin resistant. Twelve (85.7%) of the 14 children with bacterial coinfections were five years of age or older and 10 (71.4%) of the 14 children were 12 years of age or older. Five of the 17 children with confirmed pandemic influenza A (H1N1) infection had a specimen collected from a normally sterile site; bacterial cultures were negative. An increase in the number of influenza-associated pediatric deaths with bacterial coinfections was first recognized during the 2006-07 influenza season. In January 2008, interim testing and reporting recommendations were released regarding influenza and bacterial coinfections in children and are available at (http://www2a.cdc.gov/HAN/ArchiveSys/ViewMsgV.asp?AlertNum=00268).

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Influenza-Associated Hospitalizations

Laboratory-confirmed influenza-associated hospitalizations are monitored in two population-based surveillance networks: the New Vaccine Surveillance Network (NVSN) and the Emerging Infections Program (EIP).

During October 12, 2008 to June 13, 2009, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children 0-4 years old in the NVSN was 4.2 per 10,000. Because of case identification methods utilized in this study, there is a delay from the date of hospitalization to the date of report.

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During April 15, 2009 to June 27, 2009, the following preliminary laboratory-confirmed overall influenza associated hospitalization rates were reported by the EIP (rates include type A, type B, and confirmed pandemic H1N1):

Rates for children aged 0-23 months, 2-4 years, and 5-17 years were 1.4, 0.5, and 0.4 per 10,000, respectively. Rates for adults aged 18-49 years, 50-64 years, and >= 65 years were 0.2, 0.2, and 0.2 per 10,000, respectively.

^{*This value represents an age group-specific average influenza rate from October 1 to April 30 from the 2005-06, 2006-07, and 2007-08 influenza seasons.
**Note: The scales for the 0-23 month and the >= 65 years age groups differ from other age groups.}

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Outpatient Illness Surveillance:

Nationwide during week 25, 1.7% of patient visits reported through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) were due to influenza-like illness (ILI). This percentage is below the national baseline of 2.4%.

View ILINet Regional Charts | View Chart Data |View Full Screen

On a regional level, the percentage of outpatient visits for ILI ranged from 0.2% to 4.7%. Two of the 10 surveillance regions reported an ILI percentage above their region specific baseline (Regions II and X). ILI decreased during week 25 in six of 10 regions compared to week 24.

Geographic Spread of Influenza as Assessed by State and Territorial Epidemiologists:

The influenza activity reported by state and territorial epidemiologists indicates geographic spread of both seasonal influenza and pandemic influenza A (H1N1) viruses and does not measure the severity of influenza activity.

During week 25, the following influenza activity was reported:

• Widespread influenza activity was reported by 10 states (California, Connecticut, Delaware, Georgia, Hawaii, Maine, New York, Pennsylvania, Rhode Island, and Virginia).
• Regional influenza activity was reported by 11 states (Arizona, Florida, Illinois, Maryland, Massachusetts, Minnesota, Missouri, New Mexico, North Carolina, South Carolina, and West Virginia).
• Local influenza activity was reported by 15 states (Alabama, Alaska, Arkansas, Colorado, Michigan, Nebraska, Nevada, New Hampshire, Oklahoma, Oregon, Tennessee, Texas, Washington, Wisconsin, and Wyoming).
• Sporadic activity was reported by Puerto Rico and 12 states (Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Mississippi, Montana, North Dakota, Ohio, South Dakota, and Vermont).
• The District of Columbia and two states did not report (New Jersey and Utah).

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A description of surveillance methods is available at: http://www.cdc.gov/flu/weekly/fluactivity.htm

• Page last updated July 2, 2009.