CDC reports instance of probable human-to-human swine-origin flu spread

Jun 2, 2011 (CIDRAP News) – The US Centers for Disease Control and Prevention (CDC) today issued its initial assessment of the 2010-11 flu season, giving new details about swine-origin novel H3N2 infections, including probable human-to-human transmission between a Minnesota man and his child who were sick in November.

Human infections with the swine-origin flu viruses are rare, which makes a report of human-to-human transmission even more unusual.

The report, appearing in the latest Morbidity and Mortality Weekly Report (MMWR), said flu patterns during the current season showed the hallmarks of a more typical flu year compared with the previous (pandemic) season; it also said that the H3N2 virus kept its dominant position despite a late rally from the 2009 H1N1 virus.

On May 27 the CDC posted its last weekly flu surveillance report of the season, though it monitors flu activity all year.

CDC received five reports from three states of novel influenza A infections, all of them involving H3N2 swine-origin viruses, during the 2010-11 flu season, which the CDC said shows the importance of year-round testing and monitoring.

Today's report contained new details about the swine-origin H3N2 case from Minnesota first recorded in CDC's surveillance report for the week ending Dec 11. It said the man was exposed to pigs 6 days before he got sick and that a respiratory sample yielded a novel H3N2 strain. Serologic tests on his daughter, conducted 6 weeks later, showed she had been sick with the same virus, though she had no direct exposure to swine. The CDC said she probably contracted the virus from close contact with her father.

When CDC first reported on the father's infection it noted that human-to-human spread had not been detected but that investigation was ongoing. Serologic tests on other family members who were sick at the same time were negative or inconclusive, the CDC reported.

Overall, the flu season peaked in late February when 44 states reported widespread influenza activity.

Comparing the flu season that just ended with past flu seasons, the CDC said the 2010-11 season was less severe than the previous season and the 2007-08 season but more severe than 2008-09. Hospitalization rates were higher for people age 65 and older compared with the previous pandemic season and were similar to 2007-08, another season that saw the H3N2 virus dominate.

All three seasonal flu viruses circulated during the flu season, although the proportion of influenza B viruses was highest early in the season, particularly in the southeastern states. The CDC said the proportion of 2009 H1N1 viruses started to increase in January, peaking the week ending Feb 20.

So far 105 pediatric flu deaths have been reported for the 2010-11 season, which is vastly lower than last year's pandemic season but somewhat higher than the two nonpandemic flu seasons before that. Overall deaths from pneumonia and flu peaked the week ending Mar 12.

Nearly all viruses that underwent further laboratory characterization were antigenically similar to the ones covered by the seasonal flu vaccine, which meant that the vaccine was a good match with the circulating strains.

See also:

Jun 6 MMWR report

Dec 17, 2010 CDC statement

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