WASHINGTON • After struggling for months to persuade Americans to get the COVID-19 vaccine, U.S. health officials could soon face a fresh challenge: talking vaccinated people into getting booster shots to gain longer-lasting protection as the delta variant sends infections soaring again.

As early as Wednesday, U.S. health authorities are expected to recommend an extra dose of the vaccine for all Americans eight months after they get their second shot, according to two people who spoke to The Associated Press on condition of anonymity to discuss internal deliberations. That means the biggest vaccination drive in U.S. history is about to get even more extensive.

The move is being driven by both the highly contagious variant and preliminary evidence that the vaccine’s protective effect starts dropping within months.

Last week, U.S. health officials recommended boosters for some people with weakened immune systems, such as cancer patients and organ transplant recipients. If the shots are expanded as expected to other Americans, among the first to receive them could be health care workers, nursing home residents and other older people.

Some experts have expressed concern that a new campaign calling for boosters could muddle the public health message and undercut the continuing drive to win over the tens of millions of Americans who are hesitant to get their first COVID-19 shots.

Calling for third doses could discourage people who had been skeptical of the shot’s effectiveness in the first place, Lawrence Gostin, a public health specialist at Georgetown University, warned on Tuesday. “We have to really make sure that while we’re spending a lot of time and effort on third doses that we don’t undermine our campaign for first vaccinations,” he said.

The booster campaign could cause ill will toward the United States: Global health officials, including the World Health Organization, have called on wealthier nations to hold off on booster shots to ensure poor countries have enough vaccine for the initial doses.