This is not what happened. Two separate polls (by us and Opinium) done BEFORE Johnson announced his positive test on 27th March put the Tories on 54% with a 26 point lead. We had him on 72% satisfaction. Neither that, nor his ICU admission, had a discernible effect on the polls. https://twitter.com/carolecadwalla/status/1311920391842078725

It is probably true that Johnson and the Conservatives were doing so well at that point that it would have been very difficult to do much better. Had he/they been doing less well, there may have been an effect, but now we're getting into counterfactuals

In any case, so many relevant factors are different that I wouldn't confidently read across to what effect the news that #TrumpHasCovid may have on #Election2020