Source please, and without details such as cases before the parades, population age and density, crowd size, temperature, health care, and more, then your proffered correlation=causation is specious, as is this comparison with Covid-19. Yet with the Spanish flu, which unlike Covid-19, caused extensive deaths among young and fit persons, although the est. deaths have been reduced, and malnourishment, overcrowded medical camps and hospitals, and poor hygiene promoted bacterial superinfection. (https://en.wikipedia.org/wiki/Spanish_flu)
And in such a case, I support cancellations of crowed outside gatherings, besides gatherings in enclosed areas, which I noted as fostering infection, and which should be shunned in the case of Covid-19, while those quarantined need sun and fresh air each day if possible.
Let your lazy fingers do the walking.https://www.cnn.com/2020/03/15/us/philadelphia-1918-spanish-flu-trnd/index.html?utm_source=fark&utm_medium=website&utm_content=link&ICID=ref_fark