The US has reported more than 30m cases of coronavirus since the pandemic began, even as the rate of new infections has slowed precipitously following a surge in the autumn and early winter.
The nation surpassed the milestone on Wednesday, nearly 12 weeks after hitting 20m, according to Johns Hopkins University data. It had taken the US less than two months to go from 10m to 20m cases.
Brazil has recorded the second-highest cumulative total with 12.1m cases, followed by 11.7m cases in India, where daily infections have climbed in recent weeks.
In the US, new cases and hospitalisations related to Covid-19 have dropped this year, while vaccinations have risen with 46.3m Americans fully inoculated.
The US has tallied about 48,000 new cases per day over the past week, below a peak of 250,000 in early January, according to JHU. Data also showed that 37,231 people were in hospital with Covid-19 as of March 12, compared with a high of 128,312 seen at the start of the year.
The slowdown comes as more Americans get vaccinated against Covid-19. The US has administered more than 130m doses, according to the Centers for Disease Control and Prevention.
Data published on Wednesday also showed that 85.4m people have received at least one shot so far, equal to one-quarter of the total population. One-third of the adult population and 70 per cent of seniors have received one or more doses since the vaccine rollout began in December.
The number of US deaths attributed to Covid-19 stood at a total of 544,922, JHU said. New fatalities have followed other metrics on a downward trajectory, reaching daily levels last seen in November, according to the CDC.