Researchers recommend that a second booster dose should be taken by all ages of the population to prevent the onset of the disease later in life, when the chances of succumbing to the disease is high.

Says Dr Jane Seward of the Centers for Disease Control and Prevention in Atlanta, "If you’re unvaccinated and you get it later in life, there’s a 20-times greater risk of dying compared to a child, and a 10 to 15 times greater chance of getting hospitalized."

However researchers are not sure as to how long a protection does the second dose provide. Dr Seward hopes that the booster dose will be able to increase the protection against the disease to 90 to 95 percent.

Researchers followed over 350,000 cases over a period of 10 years. Of the total cases observed, 11,356 cases were of varicella. Researchers found that 9.5 percent of these cases reported the onset of chicken pox and were often characterized by formation of rash more than 42 days after vaccination.

In the report published in The New England Journal of Medicine, researchers wrote, "Children between the ages of 8 and 12 years who had been vaccinated five years or more previously were two times as likely to have moderate-to-severe breakthrough disease as were those who had been vaccinated less than five years previously."

Chickenpox is caused by the varicella-zoster virus. Symptoms include moderate fever and then characteristic spots appearing in two or three waves, mainly on the body and head rather than the hands and becoming itchy raw pox, small open sores which heal mostly without scarring.

Chicken pox viruses remain dormant in the nerves and can reactivate causing localized eruptions of shingles. This occurs particularly in people with compromised immune systems, such as the elderly, and perhaps even those suffering sunburn.

Vaccination in the United States has been taking place since 1995 and figures show that the number of cases has dropped considerably. However the main area of concern is the instances of disease occurring in patients who were already vaccinated against the disease.

In the 10-year period between 1995 to 2005, the number of cases reported has dropped by almost 85 percent, down from 2,794 cases in 1995 to just 420 in 2004. However the number of cases reported in those who have taken the vaccination had increased to 60 percent during the same period.