The mosquito-borne disease has claimed 1402 lives in India's most populous state of Uttar Pradesh since it erupted in July during the monsoon, one of the worst outbreaks in years, according to latest state figures.
"Around 70 to 75 million children in India are at a high risk from Japanese encephalitis which can hit or kill or maim them," said Marzio Babille, Unicef's chief of health for India.
"We have recommended to the health ministry to get enough vaccines by April next year before the rainy season starts, as the vaccines take about three weeks to take effect," he said.
Unicef, the UN children's fund, said the Health Ministry so far had decided to increase its production of the vaccine to just 600,000 doses from 300,000.
Most of these doses will be imported from China which has a surplus of 15 million doses available for export, Babille said.
Babille was speaking on the sidelines of a conference organised by the Global Alliance for Vaccines and Immunisation (GAVI), a public-private partnership working towards global immunisation.
He said the 70 million children most at risk were in the states of Uttar Pradesh, West Bengal, Assam, Bihar, Jharkhand and Andhra Pradesh.
The disease is transmitted from pigs to humans via mosquitoes during the monsoon rains from June to September.
Unicef said 66 million children between the age of one and five in India would be immunised against Japanese encephalitis over the next five years.
"The biggest challenge in tackling the disease is the cost and procurement of vaccines"
Unicef executive director
"The biggest challenge in tackling the disease is the cost and procurement of vaccines," said Unicef executive director Ann Veneman.
The vaccine alliance also pledged to save about 40 million children worldwide over the next 10 years from diseases such as measles, polio and tetanus for which vaccines are easily available.
GAVI chief Julian Lob-Levyt said the cost of that immunisation programme would total 10 to 15 billion dollars.
Another 20 billion dollars would be needed to give booster shots to children already immunised during the same 10-year period.
He said a third of the total 35 billion dollars would come from governments, another third from GAVI and the rest from private donors, Unicef and the World Health Organisation.
The vaccine alliance also plans to eradicate polio worldwide by 2008.