Hmm, looking at table 1, I think you are right that excluding deaths that occurred in period A from period B would only make a negligible difference.

But it might be a different case for Table 2, where the risk of myocarditis in 18-39, male, mRNA is 1.84 (1.05 - 3.21). This is nearly statistically insignificant given that the 95% confidence interval almost crosses 1.0. So, it's possible that the myocarditis risk is this group might become insignificant if the myocarditis risk in period B was not underestimated, as far as I understand. I hope that makes sense but just let me know if not.