Perhaps the best way to look at the budgetary effects of coronavirus is not that it has helped wipe out the surplus, but that the surplus has been spent on combating it. Surpluses are, in part, buffers against unexpected events — those things that muck up political plans — and in this case, it is just a matter of timing. 

Had the coronavirus erupted in, say July, the budget surplus in the piggybank would have been raided to pay for the measures taken.