The effectiveness of the varicella vaccine in clinical practice
M Vázquez1, P S LaRussa, A A Gershon
1Department of Pediatrics, Yale University School of Medicine, New Haven, Conn 06520-8064, USA.
Insights
The varicella vaccine is highly effective in preventing chickenpox, significantly reducing disease severity. This study confirms its real-world effectiveness in clinical practice for children.
Area of Science:
- Immunology
- Infectious Diseases
- Vaccinology
Background:
- The live attenuated varicella vaccine was approved in the US in 1995.
- Recommendations include vaccination for susceptible individuals aged 12 months and older.
Purpose of the Study:
- To assess the real-world effectiveness of the varicella vaccine.
- To evaluate the vaccine's impact on disease severity and incidence.
Main Methods:
- A case-control study was conducted with two controls per case, matched by age and pediatric practice.
- Active surveillance identified potential chickenpox cases; polymerase chain reaction (PCR) confirmed varicella-zoster virus.
- Disease severity was assessed during clinic visits on days 3-5 of illness.
Main Results:
- Of 330 potential cases, 243 (74%) were PCR-confirmed varicella-zoster virus.
- Vaccinated children with chickenpox had significantly milder disease (86%) compared to unvaccinated (48%).
- Overall vaccine effectiveness was 85% (95% CI, 78-90%), and 97% effective against moderate to severe disease.
Conclusions:
- The varicella vaccine demonstrates high effectiveness in clinical practice.
- Vaccination significantly reduces the severity of chickenpox illness.
Background:
A live attenuated varicella vaccine was approved for use in the United States in March 1995 and is recommended for all susceptible persons 12 months of age or older.
Methods:
To assess the effectiveness of the varicella vaccine, we conducted a case-control study with two controls per child with chickenpox, matched according to both age and pediatric practice. Children with potential cases of chickenpox were identified by active surveillance of pediatric practices in the New Haven, Connecticut, area. Research assistants visited the children on day 3, 4, or 5 of the illness, assessed the severity of the illness, and collected samples from lesions to test for varicella-zoster virus by polymerase chain reaction (PCR).
Results:
From March 1997 through November 2000, data collection was completed for 330 potential cases, of which 243 (74 percent) were in children who had positive PCR tests for varicella-zoster virus. Of the 56 vaccinated children with chickenpox, 86 percent had mild disease, whereas only 48 percent of the 187 unvaccinated children with chickenpox had mild disease (P<0.001). Among the 202 children with PCR-confirmed varicella-zoster virus and their 389 matched controls, 23 percent of the children with chickenpox and 61 percent of the matched controls had received the vaccine (vaccine effectiveness, 85 percent; 95 percent confidence interval, 78 to 90 percent; P<0.001). Against moderately severe and severe disease the vaccine was 97 percent effective (95 percent confidence interval, 93 to 99 percent). The effectiveness of the vaccine was virtually unchanged (87 percent) after adjustment for potential confounders by means of conditional logistic regression.
Conclusions:
Varicella vaccine is highly effective as used in clinical practice.
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