Unusual Rotavirus Season May Be Due to Newly Introduced Vaccine

Rotavirus activity in the ongoing 2007-2008 season appears to have started later than usual and have been less severe than during any of the previous seasons for which data are available, according to an interim report issued in today's early release edition of the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report (MMWR).

Rotavirus is the leading cause of severe gastroenteritis (vomiting and diarrhea) in infants and young children, annually causing about 410,000 physician office visits, 205,000-272,000 emergency department visits, 55,000-70,000 hospitalizations, and between 20 and 60 deaths among US children less than 5 years of age. Worldwide, rotavirus causes approximately 1,600 deaths each day among children less than 5 years of age.

Data from around the United States indicate that during the ongoing season, rotavirus activity was delayed by about three months compared with the start time for the previous 15 years. The season began at the end of February instead of November, the usual start time, and the season peaked at the end of April instead of March, the usual peak time.

Hospitalizations, emergency department visits, and physician visits were also substantially reduced at some network of medical centers conducting prospective rotavirus surveillance. The number of laboratory tests performed for rotavirus from Jan. 1 to May 3, 2008, was 37 percent lower than usual, and the percent of all tests conducted for gastroenteritis that were positive for rotavirus was 79 percent lower than usual.

The report indicates that marked changes in rotavirus activity may be due to a newly introduced rotavirus vaccine for infants. In 2006, a new rotavirus vaccine, RotaTeq (Merck & Co. Inc.), was recommended for routine immunization of U.S. infants at 2, 4 and 6 months of age. Clinical trial results indicated that this live, oral vaccine prevented 74 percent of all rotavirus cases, about 98 percent of severe cases, and about 96 percent of hospitalizations due to rotavirus.

"The changes appear to be greater than expected based on the protective effects of the vaccine alone," said Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases at CDC. "It is also possible that current levels of vaccination may be helping to decrease the spread of rotavirus to unvaccinated individuals in the community. Ongoing monitoring is needed to confirm the impact of vaccination this year and to monitor the impact of the vaccine on rotavirus disease and its epidemiology over time."

The data used in the new report were obtained from the National Respiratory and Enteric Virus Surveillance System (NREVSS) and from the New Vaccine Surveillance Network (NVSN). NREVSS is a voluntary network of U.S. laboratories that provide CDC with weekly reports of the number of tests performed and positive results obtained for a variety of
pathogens, including rotavirus.

Rotavirus is highly contagious. Large amounts of the virus are shed in the stool of infected persons and can be spread by contaminated hands and objects. Children can spread rotavirus both before and after they become sick with diarrhea, and they can pass the virus to household
members and other close contacts.