Experts have criticised the delay, saying it may have put people at risk, as earlier health warnings to the public would have meant those subsequently developing symptoms could have sought medical help sooner.
Of the 23 suspected and probable cases, all of whom are young adults and teenagers, two people have died and four were in intensive care as of Monday.
Ten reported developing symptoms between the first-known admission and the point at which UKHSA issued an alert to the public on the evening of Sunday 15 March warning there was an outbreak, according to figures released by UKHSA.
“Delaying reporting a case is indefensible,” said Prof Paul Hunter, an expert in infectious disease at the University of East Anglia. “You don’t wait for a formal diagnosis when it comes to meningitis – you report it straight away so it can be investigated.
“Not only do you want to trace the close contacts to give them preventative treatment, you also want to see if there are other cases developing.”
He added: “You had significant numbers of young people developing symptoms as the days went by, but they were unaware there was an outbreak. If they had known, they may have come forward for treatment sooner.”
He said quick treatment was essential to increase the chances of survival and to avoid life-changing disabilities, such as loss of limbs, blindness and brain injury.