A cluster of suspected whooping cough cases in Colorado was actually most likely a “pseudo-outbreak,” according to an investigation by the U.S. Centers for Disease Control and Prevention (CDC).

The cases, reported in the summer of 2009, seem to have been mistakenly diagnosed when patients’ test samples became contaminated at one medical clinic. However, the CDC says, a cluster of whooping cough cases from the winter before likely was a true outbreak.

In the U.S., most children are immunized against whooping cough with the DTaP vaccine, which is given as a series of shots starting at the age of 2 months.

But cases still occur. According to the CDC, 27,550 cases were reported nationwide in 2010, with many more probably going unreported. There were also 27 deaths, nearly all in infants younger than one year.

Many of the suspected infections were also in people who’d been vaccinated.

People diagnosed in the summer were more likely to have only small amounts of pertussis DNA in their test samples — which suggested the samples may have been contaminated.

Indeed, the CDC found pertussis DNA on surfaces at the clinic where most of the cases were seen. Of the sites the team swabbed — from nurses’ laptops to sinks to glove containers — 61 percent had detectable pertussis DNA.

The researchers suspect that pertussis DNA from vaccines contaminated clinic surfaces, and then contaminated samples taken from patients to be sent to out for testing.

Source: Reuters Health, 18th January 2012.

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