News Scan for Jul 17, 2013

West Nile recap
Flu vaccine effectiveness
Tdap vaccine protection

Warm winter, early vector findings heralded Dallas West Nile outbreak

An analysis of Dallas County's massive West Nile virus (WNV) outbreak in 2012 found that it was preceded by an unusually mild winter and favored previously known hot spots, researchers reported today in the Journal of the American Medical Association (JAMA).

The team compared the 2012 patterns with previous years. The Dallas area recorded its first WNV cases in 2002. The spike in WNV activity last summer, which resulted in 612 lab-confirmed cases and 19 deaths, followed a 5-year decline.

Researchers found that the first WNV-positive mosquito pool was detected in late May, earlier than in previous seasons, and reports of patients with symptoms began in June, a month earlier than usual. The epidemic was preceded by the mildest winter in the last 10 years.

The cases clustered in neighborhoods with high housing density in the north central part of the county, which reflected high vector indices and followed WNV patterns seen in previous years. No links were found between aerial spraying and increases in emergency department visits for respiratory symptoms or skin rash.

Knowing the characteristics of the outbreak may help future WNV control efforts, the group concluded.
Jul 17 JAMA abstract

In an accompanying editorial, Stephen Ostroff, MD, director of epidemiology at the Pennsylvania Department of Health, wrote that unusually warm winters are becoming more common and will favor spikes in WNV activity like the one seen in Dallas County. Changing weather conditions can expand risk areas and lead to longer transmission seasons.

"The tragic consequences of the Dallas West Nile virus epidemic must not be forgotten, for they serve as a cogent reminder of the need to sustain vector monitoring and prevention programs in all communities," he wrote.
Jul 17 JAMA editorial extract
Jul 16 JAMA press release


Study: Flu vaccines moderately protective against hospitalization

Seasonal flu vaccines are 37% to 48% effective in preventing hospitalizations, Australian researchers said yesterday in a PLoS One study.

The team compared influenza vaccination status in patients hospitalized with influenza confirmed by polymerase chain reaction (PCR) with hospitalized patients who had a negative PCR test for flu in an Australian sentinel surveillance system. The data covered the 2010 and 2011 Southern Hemisphere flu seasons.

Researchers were able to determine the vaccination status of 333 of the 598 flu patients, of whom 33% had received a flu vaccine. In contrast, 54% of 785 test-negative patients had received a flu vaccine, while the vaccination status was unknown for 599 test-negative patients.

From these data, the investigators calculated an overall crude vaccine effectiveness (VE) of 57%. After adjusting for age, comorbidities, and pregnancy status, the adjusted VE was 37% (95% confidence interval [CI], 12%-55%).

Further fine-tuning for clinical covariates and potential confounders using a statistical technique called a propensity scored analysis yielded an estimated VE of 48.3% (95% CI, 30.0%-61.8%).
Jul 16 PLoS One study


Kaiser study finds moderate protection for Tdap vaccine

A study of a large pertussis outbreak in California revealed that the acellular tetanus, diphtheria, and pertussis (Tdap) vaccine was moderately effective in people age 11 and older, researchers reported today in BMJ.

Worsening pertussis outbreaks in the United States and other countries have raised questions about the effectiveness of the acellular vaccine, and recent studies have shown that the vaccine's effect wanes over time. Postlicensing studies found high effectiveness for Tdap vaccines, but those studies were limited by small numbers of cases and included some people who had received the whole-cell version of the vaccine as children.

In today's study, researchers compared the Tdap vaccination status of the positive cases with that of negative controls and closely matched controls from the health system's general population. The study included all lab-confirmed pertussis cases in the Kaiser Permanente Northern California health system from January 2006 to December 2011.

The study group included 668 positive cases, 10,098 negative controls, and 21,599 matched controls. The adjusted Tdap effectiveness against pertussis was 53% (95% confidence interval, 41.9%-62%).

In their subgroup analyses the team found that the Tdap booster was moderately effective in older people who had received all whole-cell pertussis vaccines as infants and in younger people who had received all of their doses as acellular vaccine. Pertussis infection rates were highest in children ages 11 to 14 who had received acellular vaccine, which researchers said probably shows that protection from the childhood doses waned over time.

They concluded that giving the Tdap booster to people who received only acellular vaccine as children should be a priority.
Jul 17 BMJ abstract

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