Drug maker Moderna said Thursday that its coronavirus vaccine is 93% effective six months after injection.

In a statement regarding ongoing clinical trials, CEO Stephane Bancel added: "The Delta variant is a significant new threat, so we must remain vigilant."

All three of the current COVID-19 vaccines on the market in the US are believed to offer protection from the more contagious Delta variant. In rare "breakthrough" cases, in which vaccinated people get the virus, symptoms are generally mild or non-existent.

Bancel said the company is working on a booster shot that he envisions being a single-shot vaccine that would be doled once a year and would protect against COVID-19 and the basic flu.

Bancel says in clinical trials, the booster shots "demonstrate robust antibody responses to COVID-19 variants."

Meanwhile, the Food and Drug Administration (FDA) is working on granting full approval for the Pfizer vaccine by September. The Moderna and Johnson and Johnson vaccines are expected to get full approval sometime after that approval.

The three vaccines have been available on an emergency use authorization and the FDA is speeding up its timetable to grant full-use authorization.

The moves come as vaccine hesitancy in parts of the US, even vaccine hostility, is helping the spread of the Delta variant. But the White House is reporting that in areas where hospitalizations have been surging, vaccination rates are starting to tick up.