Over the next 12 months, they played around with different combinations of mutations on the surface of the spike protein until they found a set of 20 which appeared to make it particularly resilient to anything the immune system might be able to throw at it. To test this lab-grown "Frankenspike" out, they inserted it into what virologists call a pseudotype virus, one which has been engineered so it does not have enough genetic material to replicate, allowing scientists to tweak it and understand how it behaves without any risk of it escaping.