A follow-up study of 33 people who received Moderna Inc.’s Covid-19 vaccine in early trials show the antibodies it induced are still present six months after the second dose.

The U.S. National Institutes of Health said on April 7 it had begun a mid-stage study to determine the risk of allergic reactions to Covid-19 vaccines made by Moderna Inc. and Pfizer Inc.

The U.S. drug regulator gave Moderna Inc. clearance to speed up output of the company’s Covid-19 vaccine by letting it fill a single vial with up to 15 doses, with the United States banking on rapid immunization to stem the spread of the deadly virus.

The Covid-19 vaccines developed by Pfizer Inc. with BioNTech SE and Moderna Inc. are highly effective and reduced the risk of infection by 90 percent by two weeks after the second shot among healthcare personnel and first responders, according to a U.S. study released on March 29.

The U.S. government hit the accelerator on shipments of Covid-19 vaccines after a month of largely stagnant weekly deliveries, giving states the doses they say they need to finish vaccinating priority groups and open shots to all adults in the coming weeks.

The United States has administered 140,180,735 doses of Covid-19 vaccines and distributed 180,644,125 doses as of the morning of March 27, the U.S. Centers for Disease Control and Prevention said.

U.S. college students vaccinated with Moderna Inc.’s COVID-19 vaccine will be part of a new study to test mRNA-1273’s effectiveness in curbing the spread of the virus, the COVID-19 Prevention Network said on March 26.

The United States administered 121,441,497 doses of COVID-19 vaccines in the country as of the morning of March 20 and distributed 156,734,555 doses, the U.S. Centers for Disease Control and Prevention said.

The United States administered 107,060,274 doses of Covid-19 vaccines in the country and distributed 135,847,835 doses as of March 14, the U.S. Centers for Disease Control and Prevention said.

Alaska became the first U.S. state to make Covid-19 vaccines available to anyone age 16 years or older, eliminating eligibility requirements for people who work or live in the state.