Image copyright New York Times Image caption Former President Barack Obama used the drug stockpile to tackle the Ebola epidemic in West Africa in 2014
Pfizer says it will supply enough of its anti-viral medication to the US for one-third of the country’s adults.
The drug, oseltamivir, which can be used for people who have been infected with the Zika virus, was originally made for health workers in Ebola outbreaks.
The US government previously used it in 15 countries with epidemic risk.
But after two cases of Zika were confirmed in California, a need for a new supply emerged.
“The quantity of anti-viral drug supplies procured in this capacity is unprecedented for the World Health Organization’s stockpile,” a spokesman for the organisation said.
Influenza and Zika are two groups of virus which can be transmitted by mosquitoes, and can lead to severe birth defects.
The US ordered four million doses of the drug – sufficient for 10 million people – that are expected to arrive in January.
Image copyright AFP Image caption CDC scientists tested the presence of Zika virus in the Zika vaccine made by Merck
“It’s sufficient to cover a third of our adult population, and a quarter of our infants,” said Dr Chris Young, Pfizer’s senior vice president for innovation and pipeline research.
“The pharmaceutical industry is actively working on additional supplies of anti-viral drug supply to address the needs of countries worldwide.”
In the United States, the drug will be used by health officials to prevent Zika and combat influenza.
It will also be given to “high-risk” pregnant women and women with partners who are or who have had symptoms of Zika.
Image copyright AFP Image caption Initial test results suggest Zika causes severe brain damage in foetuses
In 2012, Pfizer said it would supply the medicine to West Africa and was authorised to sell it there for three years, through 2015.
It had, however, been unable to sell enough of the medicine for West Africa.
Not all European countries have agreed to import the drug.
Brazil has said it will import about a third of the drug. Other countries – which have yet to be named – will also buy the drug.
Zika was first detected in the Americas in May 2015, causing more than 50,000 to become infected and 27 deaths.
The virus, which spreads between a female Aedes aegypti mosquito that feeds on human blood and a person’s own body fluids, was first reported in Brazil, causing more than 1,500 cases of microcephaly.
At the height of the epidemic, between 3 and 4 million cases of microcephaly were recorded.
Experts believe a link between Zika infection and microcephaly was caused by the link between Zika and the Zika virus.
But last month a study found no link between Zika infection and the infant brain disorder.
According to the CDC (Centers for Disease Control and Prevention), infections with Zika during pregnancy can cause severe brain damage in the foetus.
“Although the report does not provide definitive proof of a causal association between Zika virus infection during pregnancy and microcephaly, the authors are confident that the causal association is likely,” it said.