The United States could have around 300 million excess Covid-19 shots by the end of July, health policy experts at Duke University estimated in a report Thursday, calling on the country to share doses more widely to address the stark inequality around global vaccine distribution.

The US has provided limited shipments of AstraZeneca's vaccine -- which is not yet authorized for use in the United States -- to Mexico and Canada, but US Secretary of State Antony Blinken has said it won't share shots more broadly until the country is "more confident" in its own supplies.

The US is the biggest financial donor to the global vaccine-sharing scheme COVAX, but the country has been tight-fisted with the actual vaccines it has in huge supply, while many others have none at all. Three-quarters of the world's vaccines actually administered have been in just 10 nations, which together account for under half the world's population.

"The world's wealthiest nations have locked up much of the near-term supply. At the current rate vaccines are being administered, 92 of the world's poorest countries won't vaccinate 60% of their populations until 2023 or later," wrote Dr. Krishna Udayakumar and Dr. Mark McClellan, health experts at Duke.

The report laid out a three-part plan in which the US should increase funding for COVAX, make excess doses available through the same scheme and create bilateral programs modeled on the President's Emergency Plan for AIDS Relief -- or PEPFAR -- to provide shots and support to countries in need. It could also provide the support and materials for countries to produce safe and effective vaccines on their own.

The AstraZeneca vaccine, which the US has shared, has been paused in many age groups in Europe after regulators said the vaccine was possibly linked to dozens of rare but serious blood clotting events, some of them fatal. The US suspended use of the Johnson & Johnson vaccine on Tuesday after reports of the same rare clotting event in six people. The incidence remains extremely rare, at around one in a million.

Duke University's estimate of 300 million excess doses had assumed the US would use J&J vaccine, but given the pause, their projections may be overstated. But it won't change the big picture, as the J&J shot currently makes up less than 5% of around 190 million shots currently in the US.