Yet, government scientists have found just such a gap between the reported incidence of hepatitis A in this country and the true figure. A new mathematical projection of the habits of the liver virus has found the annual number of hepatitis A cases between 1980 and 1999 was probably 270,000 -- more than 10 times the average of 26,000 cases the government recorded each year during that period.
Health officials say the undercount appears to be largely the result of undetected infections in young children who are a reservoir of virus to infect susceptible adults, and vaccinating these children should dry up the pool.
"Immunizing children will protect other children and will protect adults" from hepatitis A, says Dr. Gregory Armstrong, a medical epidemiologist at the Centers for Disease Control and Prevention and a co-author of the study, appearing in this month's issue of Pediatrics.
The shot, which is up to about 98 percent effective, is currently recommended for adults with certain risk factors for serious liver disease, such as infection with the hepatitis C virus. It's also urged for children who live in areas with at least 20 cases of hepatitis A per 100,000 people each year, or twice the national average.
Since the vaccine was introduced in the last decade, 11 states have met that criterion, Armstrong says. Routine vaccination against the virus has sharply driven down the incidence of the disease, proving the policy effective.
The CDC receives reports of some 26,000 cases of acute hepatitis A each year. The incidence is dropping by about 4.5 percent a year, thanks to improvements in sanitation.
Epidemiologists have long believed that hepatitis A -- which hops from person to person through oral-fecal transmission -- is much more common than reports would indicate. However, how much higher hasn't been clear.
Recent nationwide surveys have found that roughly 47 million Americans have been infected with the virus at some point in their life. So, Armstrong and his colleague, Dr. Beth Bell, plugged this number into a mathematical tool called catalytic modeling to estimate when those infections occurred.
The researchers concluded the true rate of infection with hepatitis A between 1980 and 1999 ranged from 190,000 to 360,000 cases a year. The most likely number was 270,000, or 10.4 times the average tally of reported cases.
More than half of those cases were in children under 10, the model projected. Another spike in cases also occurred in adults aged 20 to 29.
Life-threatening hepatitis A is rare. In children, the disease is most often indistinguishable from other viral ailments, causing fever, malaise and other vague symptoms. Only when it leads to jaundice -- the signature yellowed complexion of liver disease -- is it typically identified.
However, in adults especially, the disease is far from benign, and 85 percent of those who contract the infection become acutely ill, Armstrong says.
The latest study bolsters the argument that more people should be vaccinated against the virus, says Dr. Caroline Riely, a liver specialist at the University of Tennessee in Memphis.
"The vaccine is widely underused," says Riely, who is also associate medical director for the American Liver Foundation.
The hepatitis A vaccine is currently licensed only for children over age 2. That means it falls outside the window when they get most of their shots. But mounting evidence indicates the inoculation is safe for children as young as one year of age, which could broaden its use, Armstrong says.
What To Do
To learn more about hepatitis A and its more serious relatives, B and C, try the Centers for Disease Control and Prevention.