Vaccine for Chickenpox : Part 2

by Food and Drug Administration (FDA)

(Page 2 of 2)

Vaccine Development

Varivax's development began with a sample of varicella (chickenpox) virus isolated from the blood of a 3-year-old Japanese boy in 1972. Japanese researcher Michiaki Takahashi, M.D., attenuated (weakened) the virus by growing it in different human and animal host cells. He then tested the weakened virus in children as a vaccine against chickenpox.

His tests were successful, and in 1981, Merck obtained the rights to use the "Oka" strain (named after the 3-year-old boy) to develop its own vaccine.

Merck began clinical trials of the vaccine for safety and effectiveness. But obtaining successful test results was only half the battle for Merck to bring the vaccine to market. Making enough vaccine to meet demand was the other.

Unlike other weakened live viruses used for vaccines that spontaneously emerge from cultivated cells, varicella stays in the cell.

"In the case of varicella, we have to collect the infected cells and then break them open," explains Barry D. Garfinkle, Ph.D., Merck's vice president for vaccine quality operations. "We do this using an ultrasonic device."

The ultrasound creates heat and, in this case, creates problems. Although the varicella virus is extremely virulent in its natural environment inside the human host, once it's removed from the infected cells in the lab "varicella is very, very sensitive to elevated temperatures," says Garfinkle.

"We have to use the right amount of [ultrasound] to get the virus freed but not so much that we damage the virus," says Garfinkle. "Getting those parameters correct took us a while."

To this end, Merck built a new facility in West Point, Pa., where robots handle most of the vaccine production. Garfinkle explains that using robots allows for better control of the exacting ultrasound procedure.

Merck submitted data on Varivax's safety and effectiveness to FDA in May 1993.

In January 1994, FDA's Vaccines and Related Biological Products Advisory Committee concluded that the vaccine was safe and effective. But the committee advised FDA to address several questions before making a final decision, including how to address whether vaccinating children would shift the disease to adults and whether doctors could give Varivax at the same time as other vaccines.

In January 1995, FDA presented to the committee clinical data from the manufacturer resolving those issues.

Easy to Catch

An estimated 3.7 million Americans get chickenpox each year, with more than 90 percent of cases in people younger than 15.

Chickenpox is transmitted through fluid from broken blisters and through coughing or sneezing. A person is contagious for one or two days before the rash appears until all the lesions are dried, which usually takes four to five days. The average incubation period (the time between exposure to the virus and the onset of illness) is 10 to 21 days.

Those at greatest risk for a serious case of chickenpox are people age 13 and older, and anyone with impaired immunity. Chickenpox can also be serious for fetuses and newborn babies. When a woman breaks out in chickenpox just before or after delivery, her baby may develop a severe form of the disease; as many as 5 percent of these babies die. Infection early in pregnancy may cause the fetus to develop limb abnormalities, scarring of internal organs, or damage to the central nervous system.

In addition, "there are a fair number of complications that require hospitalization of otherwise healthy children," says Kumar.

That was the case for Brittany Evans (not her real name). The only risk factor for the healthy two-and-half-year-old was her 5-year-old sister's routine bout with chickenpox two weeks before.

"Frequently in a family, the second and third cases can be more severe," explains Kumar. Siblings generally spend more time with each other than with friends and schoolmates and this gives the virus the opportunity to infect siblings when it's most virulent.

Brittany's chickenpox broke out on Friday, March 10. By the next Wednesday she was covered with blisters and running a fever of almost 105 degrees Fahrenheit.

"She had [blisters] between her fingers so bad she couldn't get her fingers side by side," says her mother. "I couldn't rub my hand along her tummy and find a spot that didn't have a blister."

Brittany was admitted to Shady Grove Hospital in Gaithersburg, Md., on March 17 (ironically, the same day the vaccine was licensed). She spent three days there to make sure she didn't become dangerously dehydrated from the high fever. Her body's own defenses finally overcame the virus, and she recovered.

On April 10, the American Academy of Pediatrics recommended the vaccine for all healthy children between the ages of 12 months and 13 years who have not had chickenpox. For children between 12 and 18 months, the academy recommends giving the chickenpox vaccine at the same time as the first measles, mumps and rubella shot. Older children should be vaccinated at the earliest convenient time.

While some doctors still question the need for vaccinating against a generally mild disease, Kumar agrees with the academy's recommendations because of the need to prevent unforeseen complications and hospitalizations.

Brittany's mom isn't a medical expert, but she says, "I never want to go through this again. My husband and I are thinking about having another child, and I'll get the next one the vaccine, without a doubt."

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.