Children are better protected against varicella with two doses of vaccine than one, even when there is not an outbreak, according to a new study.
The finding reaffirms the AAP and Centers for Disease Control and Prevention recommendation for two routine doses — one at 12 to 15 months and another at 4 to 6 years.
Because previous studies focused on outbreak situations, researchers set out to look at community transmission of varicella outside of an outbreak. They detailed their findings in the study “Varicella Vaccine Effectiveness in Preventing Community Transmission in the Two-Dose Era” (Perella D, et al. Pediatrics. March 14, 2016, http://pediatrics.aappublications.org/content/early/2016/03/10/peds.2015-2802).
The team performed a matched case-controlled study with children in Antelope Valley, Calif., and Philadelphia and compiled data through parent questionnaires, health care providers, immunization registries and patient records. The study included 125 clinically diagnosed cases of varicella and 408 matched controls.
They found vaccine effectiveness for clinically diagnosed cases to be
- 93.6% for two doses compared with no vaccine,
- 87.5% for two doses compared with one dose, and
- 75.6% for single dose compared with no vaccine.
Vaccine effectiveness for preventing moderate/severe varicella was
- 97.9% for two doses compared with no vaccine,
- 94.1% for two doses compared with one dose, and
- 78.1% for single dose compared with no vaccine.
Among those who contracted varicella after two doses, rashes resolved more quickly and were less likely to be vesicular than those who contracted the disease after one dose. The authors said they were surprised to find less likelihood of breakthrough varicella among those who received the second dose after 6 years of age and those with more than five years between doses.
The authors noted that limitations of the study included low numbers of unvaccinated subjects and laboratory-confirmed two-dose breakthrough cases.
The study highlighted the importance of schools requiring two doses of varicella vaccine for entry. Many states do so, but not all, and researchers said the benefits go beyond the individual students receiving the vaccine.
“The reduction in community circulation of VZV (varicella zoster virus) as a result of high 2-dose coverage will also protect children who are immunocompromised and not eligible for varicella vaccination,” they said. “Additional benefits of routine childhood varicella vaccination may include reduced risk of herpes zoster among vaccinated children.”