Contagiousness of Varicella in Vaccinated Cases: A Household Contact Study. Seward JF, Zhang JX, Maupin TJ, Mascola L, and Jumaan AO. JAMA 2004;292:704-708.
Explanatory note: Sometimes, persons vaccinated against chickenpox contract the disease after exposure, which is known as breakthrough varicella. Breakthrough cases of chickenpox are often mild with less than 50 lesions.
How contagious are breakthrough varicella cases?
This study used data collected between 1997 and 2001 on chickenpox cases in Los Angeles County, California, which included information of household (family) members of the chickenpox cases.
The researchers collected detailed demographic, clinical, medical and vaccination data on every case. They also collected information on age, chickenpox history, and receipt of chickenpox vaccine on every household member of the chickenpox cases.
To establish the contagiousness of chickenpox within households, researchers calculated how many household contacts developed the disease (secondary cases) after being exposed to the primary cases. They then compared these data with the vaccination history of the primary cases to see who had been vaccinated and who had not. This allowed them to estimate the contagiousness of breakthrough varicella.
Additionally, the researchers determined how many lesions each secondary case had had (less than 50, 50-500, or more than 500 lesions).
This study found that breakthrough (previously vaccinated) cases were half as contagious as unvaccinated cases. However, breakthrough cases with more than 50 lesions were similarly contagious as the unvaccinated cases. In contrast, breakthrough cases with less than 50 lesions were only one third as contagious.
The risk of household contacts developing chickenpox from a primary case varied according to age and by disease and vaccination status of the primary case and the exposed household contacts. For example, among contacts 1 to 14 years of age exposed to unvaccinated cases, 71.5% developed chickenpox if they were unvaccinated and 15.1% if they were vaccinated. Seven percent of contacts with a history of prior chickenpox disease developed chickenpox.
Vaccine effectiveness for prevention of all disease was 78.9%; 92% for prevention of less than 500 lesions; and 100% for illnesses requiring a visit to a healthcare provider.
Persons who develop chickenpox despite having received the vaccine are less contagious than unvaccinated persons who develop chickenpox, largely as a consequence of having fewer lesions. Even under the circumstances of intense exposure in a household, chickenpox vaccine was about 80% effective in preventing all disease and reduced the number of persons with large numbers of lesions.
Household contact studies represent a very good method of estimating contagiousness of breakthrough chickenpox because within households there is the greatest likelihood for transmission to occur. Because the vaccination status of contacts was determined by the parent’s ability to recall and not from immunization records, misclassifications could have affected estimates of vaccine effectiveness.