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Varicella Zoster

The varicella-zoster virus (VZV), also known as human herpesvirus 3 (HHV-3), is one of the eight herpesviruses known to affect humans (and other vertebrates). Primary VZV infection results in chickenpox (varicella), which may rarely result in complications including VZV encephalitis. Even when clinical symptoms of varicella have resolved, VZV remains dormant in the nervous system of the host in the trigeminal and dorsal root ganglia. more...

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In about 10-20% of cases, VZV reactivates later in life to produce herpes zoster (shingles) and its associated sequelae including: post-herpetic neuralgia, zoster multiplex, myelitis, herpes ophthalmicus, or zoster sine herpete.

VZV is closely related to the herpes simplex viruses (HSV), sharing much genome homology. The known envelope glycoproteins (gB, gC, gE, gH, gI, gK, gL) correspond with those in HSV, however there is not equivalent of HSV gD. VZV virons are spherical and 150-200 nm in diameter. Its lipid envelope encloses the nucleocapsid of 162 capsomeres arranged in a hexagonal form. Its DNA is a single linear, double strand molecule, 125,000 nt long.

The virus is very susceptible to disinfectants, notably sodium hypochlorite. Within the body it can be treated by a number of drugs and therapeutic agents including aciclovir, zoster immunoglobulin (ZIG), and vidarabine.

A live attenuated VZV Oka/Merck strain vaccine is available and is marketed under the trade name Varivax. It was developed by Merck, Sharp & Dohme in the 1980s from the Oka strain virus isolated and attenuated by Michiaki Takahashi and colleagues in the 1970s. It was submitted to the U.S. Food and Drugs Administration for approval in 1990 and was approved in 1995. Since then, it has been added to the recommended vaccination schedules for children in Australia, the United States, and many other countries, causing controversy because it is only expected to be effective for about twenty years, leaving adults vulnerable to the most dangerous forms of infection by this virus, whereas getting normal chickenpox as a child typically leaves them immune for life.

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Effectiveness of varicella vaccine in children
From American Family Physician, 11/1/04 by Caroline Wellbery

Administration of varicella vaccine is recommended for children 12 to 18 months of age and for older children who have not already had chickenpox. However, varicella outbreaks in vaccinated children have occurred. In this case-control study, Vazquez and colleagues attempted to determine the extent to which age at vaccination and time since vaccination influence the effectiveness of the varicella vaccine.

The case group consisted of children 13 months to 16 years of age who developed chickenpox. In an interview conducted on day 3 to 5 of the illness, information regarding risk factors and severity of illness was collected. Polymerase chain reaction (PCR) assay was used to confirm the diagnosis of chickenpox. For each case subject, two control patients who had not had chickenpox were selected. Medical and vaccination records were reviewed for case and control patients. The effectiveness of the varicella vaccine was calculated using a standard equation.

Of the 530 case subjects, PCR assay was positive in 69 percent, negative in 23 percent, and indeterminate in 8 percent. The overall effectiveness of the vaccine was 87 percent. Chickenpox was significantly more severe in unvaccinated children than in vaccinated children. The protection of the vaccine against moderate to severe disease was 98 percent, and this overall effectiveness was not changed significantly if the child was vaccinated at 15 months or younger, although in the first year after administration, the vaccine was only 73 percent effective in children who were immunized before 15 months of age, compared with 99 percent efficacy in children immunized at 15 months or older. The effectiveness of the vaccine declined from 97 percent in the first year after vaccination to 86 percent in the second year and 81 percent after seven to eight years.

The authors conclude that at least through the first eight years after immunization, the varicella vaccine is effective, despite occasional breakthrough varicella disease, which is usually mild. A substantial, statistically significant decrease in effectiveness is evident in the second year after vaccination, with no further statistically significant decrease up to year 8 after vaccination. Effectiveness is substantially lower in the first year after immunization in children vaccinated before 15 months of age. Delaying vaccination until 15 months of age or adding a booster vaccination might solve this problem. The efficacy of the vaccine beyond eight years has not been assessed.

CAROLINE WELLBERY, M.D.

Vazquez M, et al. Effectiveness over time of varicella vaccine. JAMA February 18, 2004;291:851-5.

COPYRIGHT 2004 American Academy of Family Physicians
COPYRIGHT 2004 Gale Group

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