"... Some labs turned to mouse models of infection early in the COVID-19 pandemic only to find that mice don’t get infected with SARS-CoV-2. In order to infect cells, SARS-CoV-2 must bind to a human protein called ACE2. The human and mouse ACE2 proteins are different, and SARS-CoV-2 does not bind to mouse cells. Scientists overcame this problem by generating mice that can express the human version of ACE2 and can therefore be infected with SARS-CoV-2. When these genetically modified mice are infected by the virus, they lose weight and become sick in ways that are similar to what happens when people are infected with the virus. ..."
As you requested...
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Didn't see this last night, don't know how reliable the source is.
https://www.fetalindustry.com/post/aborted-baby-organs-used-to-humanize-mice-for-covid-19-vaccine-and-therapeutic-testing
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Another...
https://scitechdaily.com/russia-creates-custom-humanized-mice-to-test-covid-19-drugs-and-vaccines/
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From the NIH...
"... Some labs turned to mouse models of infection early in the COVID-19 pandemic only to find that mice don’t get infected with SARS-CoV-2. In order to infect cells, SARS-CoV-2 must bind to a human protein called ACE2. The human and mouse ACE2 proteins are different, and SARS-CoV-2 does not bind to mouse cells. Scientists overcame this problem by generating mice that can express the human version of ACE2 and can therefore be infected with SARS-CoV-2. When these genetically modified mice are infected by the virus, they lose weight and become sick in ways that are similar to what happens when people are infected with the virus. ..."
https://www.niaid.nih.gov/news-events/role-animal-research-mrna-covid-19-vaccine-development