Johnson & Johnson is planning to start a second large clinical trial soon for its experimental Covid-19 vaccine, this time testing whether two doses protect people from disease, rather than the single dose being tested in another large study that is ongoing.

J&J has said early-stage studies showed that a single dose of its vaccine could provide protection, and that such a shot could make a mass vaccination campaign in a pandemic emergency easier because people wouldn’t have to come back for a second injection. And a single shot would conserve supplies as J&J ramps up manufacturing enough doses to vaccinate hundreds of millions of people globally.

However, J&J also wants to test two doses of the vaccine in case the second dose serves as a booster that provides longer-lasting protection than a single dose, Joaquin Duato, vice chairman of J&J’s executive committee, said at an investor conference this week.

For the new Phase 3 trial, J&J is planning to enroll up to 30,000 people in the U.S. and several other countries, including the U.K. and Germany, according to a summary of its design posted this month on a federal database of clinical studies.

Study subjects will receive two doses of either J&J’s vaccine or a placebo, about eight weeks apart. Researchers will measure the vaccine’s efficacy by tracking whether it reduces cases of moderate to severe Covid-19 starting at least two weeks after the second dose, compared with the placebo. Study subjects will be tracked for more than two years.

A J&J spokesman said the new study hasn’t begun. J&J has said it could have initial results of the earlier large study by early 2021, with potential availability of doses for wider use soon afterward, if the federal government authorizes its emergency use.

J&J started the first Phase 3 trial in September, aiming to enroll up to 60,000 people and testing whether a single dose of the vaccine was protective. That study was paused in October after a study volunteer developed an unexplained illness. The study resumed after J&J concluded there was no evidence the vaccine caused the medical event.

Most other large trials of experimental Covid-19 vaccines, including those developed by Pfizer Inc. and Moderna Inc., are testing two-dose regimens, given three or four weeks apart. Pfizer this week reported positive interim results showing its vaccine was protective, and Moderna may report its first interim results soon.

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