The number of pertussis (whooping cough) cases registered in the state of Washington, USA, has risen considerably this year; in April 2012, the Washington State Secretary of Health declared an emergency. By 16th June there had been 2,520 reported cases of whooping cough across the state, an increase of 1,300% compared to the same period in 2011 - 37.5 cases per 100,000 people. A few days ago, Washington's epidemic passed 3,000 reported cases.

Health authorities report that the state now has the highest number of reported pertussis cases since 1942.

Health authorities say infants aged less than 12 months and children aged 10 years have been especially affected. Incidence among 13 and 14 year olds has also risen significantly, despite healthy vaccination rates in this age group for tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis (Tdap). Experts say this suggests that immunity is declining earlier than expected.

Prevention and control efforts are currently concentrated on vulnerable groups, such as young infants, people with chronic diseases, pregnant women, and those with weakened immune systems.

According to the CDC (Centers for Disease Control and Prevention), the most effective strategy for prevention of infection from whooping cough is still the pertussis vaccine.

Secretary of Health Mary Selecky, said:

"Whooping cough vaccines work but don't seem to last as long as was expected. Even so, vaccinated people who get whooping cough have milder symptoms, shorter illnesses, and are less likely to spread the disease to others. Our biggest concern is keeping babies from getting sick - and vaccination is still the best protection."

Pertussis is a serious illness for infants, who are at serious risk of severe disease and complications if they become infected - a significant proportion of infected infants have to be hospitalized. The first whooping cough vaccine dose is given to babies when they are two months old - to be fully protected, a human needs a series of five injections.

Of 185 infants aged less than 12 months who are known to have become infected, 39 were hospitalized.

Secretary Selecky, said:

"We've been working hard to slow the spread of disease and understand better what's going on.CDC has been a huge help. Analyzing the details of the whooping cough epidemic in Washington could help CDC and other states learn some things that weren't known before, and see this changing trend - and that's public health in action."

Washington health authorities say they are working closely with the CDC with the analysis of data. The two agencies are working together to try to determine how long the DTaP vaccine lasts.

According to findings from a Californian 2010 whooping cough outbreak study, the DTaP vaccine works well on children for two years. Five years after being vaccinated, children have lost approximately 70% of the vaccine's effectiveness - the longer it has been since a child's last vaccination, the higher their risk of becoming infected if they are exposed to the Bordetella pertussis bacterium, which causes whooping cough.

A study which is underway in Washington is looking into how long the TDaP booster vaccine for teens and adults last.

State Health Officer Dr. Maxine Hayes, said:

"The more we learn about whooping cough, the better we'll be able to fight this epidemic. It's a miracle a baby hasn't died in our state yet this year - it has happened before and could happen again. Although vaccine protection wears off over time, vaccination remains the best tool we have to slow the spread of this serious disease. That's why we're asking everyone to get vaccinated."

Even when the vaccine no longer provides maximum protection, it can still help reduce symptom severity, illness duration, and reduce the rate at which the disease spreads.

A CDC MMWR report, which covered the period January 1 to June 16, 2012, in Washington state, informed:
  • There were a total of 2,520 pertussis cases, of which 83.4 were confirmed in a lab, and 16.6% were epidemiologically linked, while 451 were probable cases

  • There were 1,300% more cases reported in 2012 than in 2011 for the same period

  • Whooping cough cases were reported in 32 out of 39 counties

  • Across the state, incidence was calculated at 37.5 cases per 100,000 people, ranging from 414.9 in one county to 4.9 in another

  • Infants and children aged 10, 13 and 14 years had the highest incidence

  • The highest incidence was among Hispanics, 53.1 per 100,000 compared to 24.6 per 100,000 among non-Hispanics

  • Of the 155 infants (aged less than 12 months), 21.9% (34) were hospitalized. 41.2% of hospitalized infants were less than two months old.

  • No deaths caused by whooping cough have been reported

  • 75.8% of patients aged from 3 months to 10 years were up-to-date with their childhood diphtheria and tetanus toxoids and acellular pertussis (DTaP) doses

  • 43.1% of patients aged 11 and 12 years and 77.2% of 13 to 19 year olds were up-to-date with their TDaP booster shots.

  • 93.2% of children aged from 19 to 35 months had received three or more doses, while 81.9% had received four or more in 2010

Whooping cough in the USA

Whooping cough is endemic (common, widespread) in the USA. After reaching lows in the 1970s, pertussis incidence had been gradually increasing across the country.

There were 27,550 reported cases of pertussis nationwide in 2010. Annual counts from 2012 have overtaken those from the previous 5 years for the first six months of each year. Increased rates among 13 to 14 year-olds have occurred across the country.

Reported cases of whooping cough (pertussis) in the USA (Source: CDC)
  • 2000 - 7,867
  • 2001 -7,580
  • 2002 - 9,771
  • 2003 - 11,647
  • 2004 - 25,827
  • 2005 - 25,616
  • 2006 - 15,632
  • 2007 - 10,454
  • 2008 - 13,278
  • 2009 - 17,550
  • 2010 - 27,550
  • 2011 - 15,216
  • 2012 - numbers are expected to be considerably higher than anything seen this century so far
Written by Christian Nordvist