07/15/2015 02:09 EDT | Updated 07/15/2016 05:59 EDT

Big drop in chickenpox cases after Ontario began public vaccine program: study

TORONTO - Ontario's publicly funded chickenpox vaccination program appears to have dramatically reduced the number of children who get infected with the virus, researchers say.

In a study that looked at 20 years of data, Public Health Ontario found the number of children who visited a doctor or an emergency room due to chickenpox dropped 71 per cent after the public immunization program began in 2004, compared to two earlier periods. Hospitalizations also fell 59 per cent.

The study looked at health-care usage related to chickenpox among kids under 18 from 2004 to 2011. The numbers were compared to the period from 1992 to 1998, when there was no vaccine, and 1999 to 2003, when the shot was available but had to be paid for out of parents' pockets.

"This is a vaccine program that very clearly is working," said lead author Dr. Anne Wormsbecker, a pediatrician and epidemiologist at Public Health Ontario.

"So it looks like from our study that kids are not getting as much chickenpox ... and fewer kids are being hospitalized for complications of chickenpox.

"So that's really a good news story."

Chickenpox is a highly contagious illness caused by the varicella zoster virus, which produces an itchy, blister-like rash and lasts about five to 10 days. Other symptoms include fever, fatigue and headache.

The virus spreads in the air when an infected person coughs or sneezes, but can also be contracted by touching or breathing in the virus particles from chickenpox blisters, according to the U.S. Centers for Disease Control.

"Chickenpox is usually a mild, self-limiting illness," said Wormsbecker.

"However, chickenpox in some cases — and we can't predict in which cases — can result in very serious complications, including infections of the skin and deeper tissues, the most serious being necrotizing fasciitis."

Those skin and tissue infections are caused by different types of bacteria, likely introduced when a child scratches a blister, she said.

Necrotizing fasciitis, or flesh-eating disease, is a potentially life-threatening infection caused by the group A streptococcus bacterium and can be severe enough to require surgery or, in some cases, limb amputation.

But the virus itself can also cause complications, by infecting the lungs, brain or bloodstream and organs, resulting in disability or death in rare cases.

"So that to me is a reminder that it's important to have the chickenpox vaccine, and I'm really happy to see that our vaccine is working," said Wormsbecker.

Dr. Joan Robinson, who was not involved in the study, said that when the varicella vaccine was first introduced, there were doubts as to whether it was really needed.

"Many people regarded chickenpox as being just a rite of childhood (and) it's usually quite a benign disease," said Robinson, an infectious disease specialist at Stollery Children's Hospital in Edmonton.

"But it's been recognized by physicians that occasionally it does cause quite severe disease, even in children who have no known risk factors for having severe disease," she said, noting that an estimated 1,000 children in Canada were hospitalized each year due to chickenpox complications prior to the vaccine's development.

"This study really nicely demonstrates that the vaccine has had an impact."

The research, published online Wednesday in the journal PLOS ONE, showed that between 1992 and 2011, there were more than 600,000 physician office visits, 55,500 emergency department visits and 2,700 hospitalizations for chickenpox among Ontario children.

In 1994, before the vaccine became available, visits to doctors' offices due to chickenpox were 25 per 1,000 children. In 2011, seven years after the publicly funded varicella vaccination program began, that figure dropped to 3.2 per 1,000 children.

The rates of hospital ER visits and hospitalizations saw similar declines, and chickenpox-associated skin and soft tissue infections declined significantly, especially in kids under 12.

Initially, children were given one dose of the varicella vaccine. But in 2011, doctors began administering two shots — at 15 months old and between age four and six — after research showed the double-dose regimen was better at preventing outbreaks in schools and other settings where groups of children come in close contact.

In Ontario, almost 78 per cent of five-year-old kids are known to have had at least one shot of the vaccine.

Most provinces cover the cost of vaccination to prevent chickenpox, and these programs even help those who haven't been immunized because of the "herd immunity" phenomenon — which makes it more difficult for the virus to keep circulating and infecting people within the community.

Such herd immunity is also important for helping to prevent the spread of chickenpox due to imported cases, such as the Mexican soccer player who was diagnosed with the virus and isolated herself shortly after arriving in Toronto to compete in the Pan Am Games.

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