US President Joe Biden welcomed the announcement that US regulators authorised a third Covid-19 vaccine.
The Food and Drug Administration said on Saturday that it granted an emergency-use authorisation for the single-dose Johnson & Johnson vaccine for people 18 and older.
But Mr Biden issued a warning that the country could not become complacent.
"This is exciting news for all Americans and an encouraging development in our efforts to bring an end to the crisis," he said.
"But we cannot let our guard down now or assume that victory is inevitable."
The FDA announcement came after the US reported 500,000 Covid-19 deaths and at a time when health officials are raising concerns that a recent decline in new cases may be stalling.
The Johnson & Johnson shot is highly effective at preventing severe Covid-19, with no serious side effects, agency staff said in a report on Wednesday.
On Friday, a committee of outside advisers to the FDA voted unanimously that the vaccine's benefits outweigh any risks. Johnson & Johnson is prepared to ship between three million and 4 million doses across the US.
“We’re in a race between the virus mutating, new variants coming out that could cause further disease, and stopping it. We need to get this vaccine out,” said Jay Portnoy, director of the division of allergy, asthma and immunology at Children’s Mercy Hospital, who supported an emergency authorisation.
The shot is expected to be easier to distribute and administer than the Pfizer-BioNTech vaccine and the Moderna shot, which were cleared by the FDA late last year.
To reach full effectiveness, those shots require two doses to be given several weeks apart. They must also must be kept at very cold temperatures, while the Johnson & Johnson vaccine can be stored in a fridge for long periods of time.
The inoculation drive in the US accelerated in recent weeks, with more than 70.5 million doses given as of Friday. However, Mr Biden said the rise of new variants still posed a threat.
"As I have said many times, things are still likely to get worse again as new variants spread, and the current improvement could reverse," Mr Biden said on Saturday.
The Johnson & Johnson vaccine is one of the few tested in clinical trials against a variant and it had a 64 per cent efficacy rate at preventing moderate-to-severe disease in South Africa.
The company said on Friday that it was developing a second-generation vaccine that would focus on the South African variant and would be ready to start Phase 1 trials this summer.
Its current vaccine uses a common cold virus known as adenovirus type 26 to introduce coronavirus proteins into cells in the body and trigger an immune response.
Both the Pfizer and Moderna vaccines, which are based on new messenger RNA technology, showed higher efficacy rates in their pivotal trials when tested using two doses versus J&J's single-shot vaccine.
But experts cautioned against close comparisons because the trials had different goals and the Johnson & Johnson trial was conducted while the more contagious new variants were circulating.
The company is testing a two-dose version of its vaccine, with results expected this summer.
