New UK COVID-19 vaccine recommendations say 'it is reasonable' for people to mix and match different shots, even though there's not yet evidence that works

• The UK has authorized two different coronavirus vaccines: one from Pfizer/BioNTech, and another from AstraZeneca/University of Oxford.
• Both require two doses, given several weeks apart.
• The British government is suggesting that people could mix and match their two COVID-19 shots, if need be.
• There's a trial going on to see how well mixing different shots would work, but there are no results yet.

With both AstraZeneca and Pfizer's shots now authorized for emergency use, the UK has two different COVID-19 vaccines available to fight the pandemic. Both of them require people to get two shots, several weeks apart.

But, if people forget which one they got first, or, if providers run out of one kind or the other, the UK government is now saying: no worries.

In guidance freshly updated on New Year's Eve, one day after AstraZeneca's vaccine was authorized for use in the UK, the British government suggested that people may mix and match their two COVID-19 shots - and government experts even think there's a chance people may get better protection from coronavirus infections in that way.

If "the same vaccine is not available, or if the first product received is unknown, it is reasonable to offer one dose of the locally available product to complete the schedule," the UK's new advice for providers reads.

However, both experts and government officials agree that mixing two vaccines together in the hopes of providing people with more robust protection from coronavirus infections is still a risky, untested strategy.

The 'Wild West' of vaccination campaigns

Even the British government writes in its new guidance that "there is no evidence on the interchangeability of the COVID-19 vaccines."

"We're kind of in this Wild West," Dr. Phyllis Tien, an infectious disease physician at the University of California, San Francisco, told the New York Times. "None of this is being data driven right now."

It's possible that mixing different vaccines together could provide people with more robust protection from infection - and government officials in the UK are launching a so-called 'mix and match' trial, to find out if that's the case.

"The idea is that you can maximize the strength of that immune response to protect people," Kate Bingham, chair of the UK's vaccine task force, said during a recent briefing.

Britain has cleared two different vaccines for emergency use so far. First, Pfizer/BioNTech's vaccine was given the green light in the UK on December 2, and then AstraZeneca/University of Oxford's followed, on December 30. Both of these vaccines were designed to be administered as two shots, given several weeks apart. But they are not the same kind of inoculation, nor were they designed to be taken together.

Pfizer's vaccine is a new kind of messenger RNA vaccine, while AstraZeneca is using viral vector technology. Working together, it's possible they could provide people with a solid one-two punch of both good cellular (from AstraZeneca) and good antibody (from Pfizer) virus response, as Business Insider's Kate Duffy recently reported.

"Antibodies block the uptake of viruses into cells, and the cellular T-cells identify those cells that have been infected and take them out," Bingham said. "You ideally want to have both."

But John Moore, a vaccine expert at Cornell University, wants more evidence that strategy can really work before it is recommended.

Moore told the Times that officials "seem to have abandoned science completely now and are just trying to guess their way out of a mess."

Less than 2% of the population in the UK is vaccinated, with a fast-spreading new variant on the loose

The UK has recently been slammed by more coronavirus infections, a surging wave fueled in part by what is suspected to be a fast-spreading coronavirus variant. The new variant, called B.1.1.7, is not more deadly, and experts expect vaccines will be successful at fighting it, too.

The country is now pushing to get as many people vaccinated as possible with one COVID-19 vaccine dose, before administering them their second booster shot.

"At this stage of the pandemic, prioritizing the first doses of vaccine for as many people as possible on the priority list will protect the greatest number of at risk people overall in the shortest possible time," UK officials said in a statement on Tuesday.

Fewer than 1.5% of people in the UK have gotten shots so far, according to Bloomberg's COVID-19 vaccine tracker.