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UK researchers and officials are preparing to infect human volunteers with the delta variant of the coronavirus — on purpose — in a controversial but important "challenge" study that will help them directly measure the impact of vaccines and treatments.

Researchers in the UK have already exposed about 40 human volunteers to the original variant of SARS-CoV-2, the virus that causes COVID-19, so that they can test vaccines and treatments in a head-to-head, controlled setting. But now that the delta variant has infected its way to international prominence, the researchers find themselves struggling to respond in kind.

To collect that invaluable data, The Wall Street Journal reports that scientists at laboratory service company hVivo are almost finished cooking up a batch of the delta variant cultivated specifically for human experimentation.

"Delta has reduced the relevance of the first virus," respiratory-viral infection expert Garth Rapeport, who helped set up the human experiments, told the WSJ.

Ironically, the delta variant that's rapidly surged around the world and caused a horrific new wave of infections in the US is having a hard time growing in the lab.

"Delta has been more difficult," hVivo chief scientific officer Andrew Catchpole told the WSJ. "Not all clinical samples grow as readily in cell cultures."

Part of that challenge is the fact that the researchers have to ensure that any mutations in their lab-grown samples are a perfect mirror of the variant that's spreading through the outside world.

"It’s not that dissimilar to making vaccines," Rapeport told the WSJ. "It has to be highly controlled and regulated."