Dec 14, 2012 (CIDRAP News) – Influenza activity continued its early-season upward trend in the United States last week, while the flu season showed some signs of getting under way in Europe. In addition, US investigators found another case of swine-origin H3N2 flu, this one in a Minnesotan who has recovered.
The share of US respiratory specimens that tested positive for flu reached 28.3% last week (2,172 of 7,663), up from 20.7% a week earlier, the Centers for Disease Control and Prevention (CDC) reported in its weekly update. The proportions varied from 7.9% in California and Arizona to 47.3% in the Midwest.
The CDC said the proportion of all medical visits attributed to influenza-like illness (ILI) was 2.8%, exceeding both the national baseline (2.2%) and last week's 1.9%.
Seven of 10 regions reported ILI activity higher than the regional baseline, the CDC reported. Eight states had high activity, compared with four last week, and two had moderate activity. Another 9 states and New York City reported low activity, and 31 states listed activity as minimal.
The geographic extent of flu as reported by state epidemiologists was widespread in 18 states, up from 8 a week earlier, the CDC said. Seventeen states reported activity as regional, 11 states and the nation's capital as local, and Guam and four states as sporadic.
The agency said one pediatric flu death, linked to a type B virus, was reported last week, versus three deaths the week before. The season's total has reached six.
Meanwhile, the share of US deaths attributed to pneumonia and flu last week was 6.4%, which is below the epidemic threshold of 6.9% for the week and just below the previous week's 6.5%.
Of flu viruses that were typed last week, 76.2% were influenza A and 23.8% were influenza B. H3N2 continued to predominate among type A viruses: of 805 isolates that were subtyped, 791 were H3N2 and 14 were 2009 H1N1.
So far this season, nearly all of the H3N2 viruses and all the 2009 H1N1 isolates were good matches for the strains in this year's vaccine. For type B, 66.3% of tested isolates have matched the Yamagata lineage in the vaccine, while 33.7% have belonged to the Victoria lineage.
Variant H3N2 case
The swine-origin H3N2 case (variant H3N2, or H3N2v) was detected through routine surveillance and involved a person who was exposed to pigs at a live animal market, said Ruth Lynfield, MD, Minnesota's state epidemiologist. The market was one of two such businesses in the St. Paul area.
Lynfield said the patient, who got sick in November, was treated as an outpatient and has fully recovered. She declined to give other details about the patient or the illness. Monitoring of the person's contacts has picked up no additional cases.
The CDC said the case is the second H3N2v illness detected in this flu season, which began Sep 30. But a total of 312 variant flu cases have been reported in 11 states since July, including 308 H3N2v, 3 H1N2v, and 1 H1N1v, the agency said. The vast majority of those have been in people, mostly children, who were exposed to pigs at county and state fairs.
Lynfield noted that Minnesota had two confirmed variant flu cases and one probable case that were linked to live animal markets this past summer, plus one such case in an earlier year. She observed that flu infections are common in swine.
A University of Minnesota researcher who is involved in a study on swine flu at Minnesota's live animal markets said some pigs at the market in question tested positive for flu around the time of the H3N2v case. The start of the study predated the human case.
"We did have samples that were positive. We've been finding them as part of the overall study," said Montse Torremorell, DVM, PhD, who is an associate professor and holds the Leman Chair in Swine Health and Productivity at the university. She said H3N2 and H1N1 swine flu strains have been found.
Torremorell said the findings were not unexpected, since flu is common in pigs. Some of the infected pigs have shown no signs of illness, while others have had clinical illness, she noted. Given how common swine flu is, the risk of transmission to people is low, she said.
The Minnesota case-patient was visiting the market at the time of exposure, not working there, Torremorell said.
Flu starting to pick up in Europe
In Europe, overall flu activity remained low last week, the European Centre for Disease Prevention and Control (ECDC) reported today. But for the first time this season, 15 of the 24 reporting countries cited sporadic geographic spread, and eight of them saw "increasing trends." Another eight countries reported no activity.
About 13% (13.3%) of sentinel respiratory specimens tested positive for flu, which marks the second consecutive week with a notable increase in the percentage of positives, the agency said.
So far this season, 53% of the flu isolates in sentinel specimens have been type A and 47% have been type B. Among the type A viruses, 53% have been H3 and 47% have been H1, the ECDC said.
The agency said the circulating viruses remain well-matched to the strains in this year's vaccine.
Dec 14 CDC FluView report
Dec 14 ECDC flu update
Nov 30 CIDRAP News story covering previous H3N2v case