The chief executive of AstraZeneca, which is developing a leading coronavirus vaccine with Oxford University, said it is too early to deliberately expose trial participants to the pathogen, but it may become an option if ongoing tests hit a snag.

The United States plans a massive testing effort involving more than 100,000 volunteers and a half dozen or so of the most promising vaccine candidates in an effort to deliver a safe and effective one by the end of 2020, scientists leading the program told Reuters.

Oxford University and AstraZeneca are recruiting around 10,000 adults and children in Britain for trials of an experimental coronavirus vaccine, a day after receiving U.S. backing worth up to $1.2 billion.

The United States secured almost a third of the first one billion doses planned for AstraZeneca’s experimental COVID-19 vaccine by pledging up to $1.2 billion, as world powers scramble for medicines to get their economies back to work.

Pregnant women are no more likely to become severely ill with COVID-19 than other women, according to a preliminary study in Britain, but most expectant mothers who do develop serious illness tend to be in the later stages of pregnancy.