Throughout the pandemic, masks have ranked among the most contentious public health measures in the United States, symbolizing a bitter partisan divide over the role of government and individual liberties.

Now, with a new variant of the coronavirus rapidly spreading across the globe, masks are again the focus of conflicting views, and fears, about the course of pandemic and the restrictions required to manage it.

Three scientific studies released on Monday offered fresh evidence that widely used vaccines will continue to protect people against the coronavirus for long periods, possibly for years, and can be adapted to fortify the immune system still further if needed.

Most people immunized with the mRNA vaccines may not need boosters, one study found, so long as the virus and its variants do not evolve much beyond their current forms — which is not guaranteed. Mix-and-match vaccination shows promise, a second study found, and booster shots of one widely used vaccine, if they are required, greatly enhance immunity, according to a third report.