(NEXSTAR) — Booster shots of the COVID-19 vaccine are set to roll out to the general public starting Sept. 20, but that doesn’t mean everyone is going to get one right away. The Biden administration is suggesting people wait eight months since their second dose before going for a third.

With the delta variant surging (and more variants on the way), and with vaccine supply ample in America, why the wait?

The eight-month waiting period has more to do with logistics than anything else, says Dr. Peter Chin-Hong, an infectious disease expert at the University of California, San Francisco.

“The idea is that the Biden administration was basing that eight month interval based on the science from Israel showing that immunity starts waning at around the six-month mark and not wanting everyone to rush out and get it at the same time,” Chin-Hong says.

The “eight month” number doesn’t have any real significance (except that it’s a lucky number in many Asian cultures, Chin-Hong jokes). But spacing out the doses is important.

“The immune system generally does better in terms of ‘remembering’ if you wait a few months before the last shot,” he explains. “If you get all three shots at the same time, or really close to each other, you may not be doing what you set out to do – i.e. train the immune system to remember (how to fight a virus) for many years.”

The interval between second and third doses may vary by vaccine type, adds Dr. Bob Wachter, chair of the Department of Medicine at UCSF. “For example, if Moderna is holding up better than Pfizer, perhaps we don’t need third Moderna shots just yet. And there is some question whether the original Moderna dose was higher than it needed to be – (then) perhaps the booster can be a lower dose.”

Those are questions still being sorted out by the Food and Drug Administration, whose scientific advisers are set to meet on the issue of booster shots on Sept. 17. It looks like the FDA may just give Pfizer’s third doses the green light at first, with Moderna and Johnson & Johnson approval coming down the road.

There’s enough data to support Pfizer’s efficacy waning at around six months post-vaccination to aim for a booster shot around then, says Wachter. At this point, we’re just waiting for the formality of an FDA approval.

“It’s kind of arbitrary whether you choose six months – my personal fave as it’s easy to remember – or 7.5 months or 8.99 months,” says Chin-Hong. All that matters is that it’s several months after your first round of vaccinations.

A consensus on “optimal timing” may emerge in the future, and the answer may not be the same for people of different ages and risk categories.

“There’s been talk about giving it after eight months; some have said after six months; some have said after a year,” Dr. Clay Dunagan, with the St. Louis Metropolitan Pandemic Task Force, recently told Nexstar’s KTVI. “Those are really issues that are being worked out looking at a variety of sources of information from countries that have started to use boosters as well as additional studies that the pharmaceutical companies have done.”

“Whatever the right answer is, it is not an emergency to run out and get a booster for the general population as hospitalizations and deaths are still spectacularly averted by vaccines,” Chin-Hong says.

On Wednesday, the head of the World Health Organization urged wealthy countries with ample vaccine supplies to pause booster shots through the end of the year so that doses could be made available to poorer countries.