Lawsuits are only allowed for harms to a particular private individual's health that actually arises from the conduct of the person sued. You can't sue because there was an outbreak in your city, you have to show individualized harm to you.
To prevail you would have to make out a "negligence" claim. The elements of this claim are (1) a duty, (2) that was breached, (3) by someone with injuries, (4) that were caused by the breach of duty.
There are two kinds of duties. One is the general duty to use the care of a reasonable person to prevent foreseeable harm to others (a breach of this duty is called "negligence"). The other is to obey statutes and regulations and ordinances intended to protect people like the person injured from the harm suffered.
If someone doesn't violate a statute, regulation, or ordinance, then one has to show that it as negligent to carry out the event.
There would also be an affirmative defense that the fault was entirely that of the victim by attending the event knowing of the risk.
I wouldn't rule out negligence liability for event organizers, but it is more likely that they would be sued not for holding the event, but for failure to use reasonable care in how it was conducted (e.g. not having face masks and hand sanitizer on hand).