US nuclear posture explicitly allows the use of nuclear weapons in response to a massive biological attack by a nuclear power like China. Deterring such an attack is an express mission of our nuclear forces.
If we really believe and can demonstrate that the C-19 pandemic was a deliberate act of biological warfare by China, then it would seem to be that a massive nuclear response is almost imperative to avoid forfeiting the credibility of our nuclear deterrent.
Biological warfare is not an accusation to level lightly.
The only issue with the retaliation is an American or Americans were the ones who actually funded the research. It may have been “designed” in China but who actually released it?