WASHINGTON, United States (AP) — The United States Food and Drug Administration (FDA) is wrestling with whether and when to offer another dose of the single-shot Johnson & Johnson COVID-19 vaccine, while a new study out yesterday raises the prospect that using a different brand as the booster might work better.

In an online review posted yesterday, FDA scientists didn’t reach a firm conclusion about whether there’s enough evidence for J&J boosters, citing shortcomings with the company’s data and little information on protection against the extra-contagious Delta variant of the coronavirus.

The review comes ahead of meetings today and tomorrow when an FDA advisory panel will recommend whether to back booster doses of both the J&J and Moderna vaccines.

That’s one step in the Government’s vaccine review process: Next week, the FDA will make a final decision on authorising those boosters and then the Centers for Disease Control and Prevention will debate who actually should get them.

Adding to the complexity is whether it’s OK to use a booster that’s a different brand than someone’s initial shots. Preliminary results of a US Government study suggest that mixing and matching boosters will work at least as well — and maybe far better for J&J recipients.

Those people had a stronger immune response if they got either a Moderna or Pfizer shot as their booster than if they received another dose of the J&J vaccine, according to results posted online yesterday. Mix-and-match is also up for discussion by the FDA panel this week.

Health authorities say all the vaccines used in the US continue to provide strong protection against severe disease or death from COVID-19. But amid signs that protection against milder infections may be waning, the Government already has cleared booster doses of the Pfizer vaccine for certain people starting at six months after their last shot.

Aiming for uniform recommendations, Moderna likewise asked the FDA to clear its booster dose at six months. But J&J complicated the decision by proposing a second shot over a range of two to six months.

FDA reviewers wrote that a study of the two-month booster plan suggests “there may be a benefit,” while pointing to only small numbers of people who got another shot at six months instead.

Overall, the J&J vaccine “still affords protection against severe COVID-19 disease and death,” the FDA’s reviewers concluded. But data about its effectiveness “are consistently less” than the protection seen with Pfizer and Moderna shots.

For its part, J&J filed data with the FDA from a real-world study showing its vaccine remains about 80 per cent effective against hospitalisations in the US.

J&J’s single-dose vaccine was highly anticipated for its one-and-done formulation. But its roll-out was hurt by a series of troubles including manufacturing problems and some rare but serious side effects including a blood clot disorder and a neurological reaction called Guillain-Barre syndrome. In both cases, regulators decided the shot’s benefits outweighed those risks.

Rival drug makers Pfizer and Moderna have provided the vast majority of US COVID-19 vaccines. More than 170 million Americans have been fully vaccinated with those companies’ two-dose shots while less than 15 million Americans got the J&J shot.

In the meantime beleaguered business owners and families separated by COVID-19 restrictions rejoiced yesterday after the US said it will reopen its land borders to non-essential travel next month, ending a 19-month freeze.

Travel across land borders from Canada and Mexico has been largely restricted to workers whose jobs are deemed essential. New rules will allow fully vaccinated foreign nationals to enter the US regardless of the reason starting in early November, when a similar easing of restrictions is set for air travel.

By mid-January, even essential travellers seeking to enter the US, such as truck drivers, will need to be fully vaccinated.

Shopping malls and big box retailers in US border towns whose parking spaces had been filled by cars with Mexican license plates were hit hard by travel restrictions.

San Diego Mayor Todd Gloria said the economic impact was hard to quantify but can be seen in the sparse presence of shoppers at a high-end outlet mall on the city’s border with Tijuana, Mexico. The decision comes at a critical time ahead of the holiday shopping season.

In Del Rio, Texas, Mexican visitors account for about 65 per cent of retail sales, said Blanca Larson, executive director of the chamber of commerce and visitors bureau in the city of 35,000 people.