GIARDINI NAXOS, SICILY, ITALY -- April 11, 2003 -- The introduction of a varicella vaccine in the United States in 1995 was followed by a decrease in the rate of relatively rare hospitalizations for the serious complications of the infection, researchers reported here.
 

Researchers looked over data from three states -- California, Connecticut and Washington -- as well as national statistics and noticed a fall from stable levels of hospitalization for varicella in each of the states that appears to correspond to the level of vaccination coverage in the state.
 

"The data suggested that varicella hospitalization rates were stable before introduction of varicella vaccine and decreased significantly in the three studied states after vaccine introduction, without high rates of vaccine coverage," said Alexander Nikas, an epidemiologist with Merck & Co., Inc., West Point, Pennsylvania, in a poster presentation April 11th at the 21st Annual Meeting of the European Society for Paediatric Infectious Diseases.
 

In California, with vaccine coverage at 74% for children under 2 years of age, the hospitalization rate for varicella complications was falling at the rate of 21.3% a year by the year 2000, he said. The hospitalization rate in California fell from about 6.1 cases per 100,000 people to about 1.6 cases per 100,000.
 

In Connecticut, with vaccine coverage at 72% for children under 2 years of age, the hospitalization rate for varicella complications was falling at the rate of 19.7% a year by 2000, he said. The hospitalization rate in Connecticut fell from about 5.2 cases per 100,000 people to about 1.6 cases per 100,000.
 

In Washington State, with vaccine coverage at 48% for children under 2 years of age, the hospitalization rate for varicella complications was falling at the rate of 11.7% a year by 2000, he said. The hospitalization rate in Washington fell from about 5.9 cases per 100,000 people to about 2.1 cases per 100,000.
 

According to data gleaned from the US National Hospital Discharge Survey, with national vaccine coverage at 65%, the hospitalization rate fell from about 7.2 cases per 100,000 to about 2 cases per 100,000 by 2000. However, Dr. Nikas said that due to the nature of collection data, statistical analysis of the national figures was not performed.
 

He said the decline in rates might represent herd immunity to varicella, despite high rates of coverage.
 

He said the study was limited by only analyzing hospitalizations; by only looking at state level and not individual vaccination levels; by changes in varicella incidence and severity but also by medical practice coding.
 

The study was supported by Merck Research Laboratories, manufacturer of Varivax varicella vaccine.