This outbreak has been called unusual and unprecedented because of the number infected in such a short space of time.

The worst seems to be over with no newly confirmed cases for a week, but there is still an unsolved mystery - why did this outbreak occur?

The explanation is critical because it will tell us whether teenagers and young adults are at greater risk of meningitis than previously thought, and whether this was a deeply unlucky one-off or if it could happen again.

We're not facing another Covid.

Meningitis is very rare.

However, cases of bacterial meningitis do not normally happen like this.

Only rarely do they get through the barriers at the back of the nose and throat to invade the body and cause meningitis and sepsis.

This happens so infrequently the disease normally appears as random, isolated one-off cases.

The cluster of cases suggests a lot of people were infected at the same time, but nothing obviously unusual took place in Kent.

There has always been an increased risk of meningitis in teenagers and young adults, particularly university students who are 11 times more likely to have meningitis than non-students, although it is still rare even on university campuses.

Moving around the country and socialising with new people exposes us to different strains of meningococcal bacteria for the first time and in turn the risk of meningitis.

But this does not explain the Kent outbreak.

Neither does a packed nightclub where people are sharing drinks and vapes as this happens up and down the country every week.

So a lot of the focus is on changes to the bacterium and whether people have less immunity.

"All the individual circumstances don't seem particularly exceptional, which is why there will be a rush to understand this particular bacteria and whether there's something new about the type that has been found in this outbreak," Gil told BBC Radio 4's Inside Health.

It shows the strain has been circulating in the UK for five years and hasn't caused this kind of outbreak before.

The report also identified "multiple potentially significant" mutations that may alter the bacterium's behaviour.

Quite how important they are will require more work.

It is also unknown how widespread this particular variant is.

Did it emerge in Kent and has since been wiped out by the mass use of antibiotics in response to the outbreak?

Or will studies planned to assess the types of meningococcal bacteria people are harmlessly carrying reveal it is more widespread?

This could be due to generational shifts in the nature of teenage life.

These bacteria are spread by close physical contact.

Could spending less time together in person, including Covid restrictions, and more time socialising through a screen lead to lower levels of immunity?

Studies on the blood of healthy 15 to 22-year-olds will assess antibody levels to see if current young adults have been left more vulnerable.

Any attempt to explain what happened in Kent is still mired in uncertainty.

The best guesses of scientists chasing the answer is that it took a combination of the bacteria, immunity levels and the club setting to allow it to spread.

That makes predicting the future hard.

The UK Health Security Agency has attempted to anticipate what could happen in the next six months, but has only "low confidence" in any of its forecasts.

It says more cases or clusters is a realistic possibility - meaning a 40-50% chance of it happening - but there is only a remote chance, less than 5%, of widespread outbreaks.

It leaves a difficult challenge in deciding what should be done – particularly whether teenagers need extra protection by being offered a meningitis B vaccine.

A MenB vaccine was introduced in the UK in 2015, but only for those most at risk – babies and toddlers.

There is a second, smaller, peak in risk for late teens and in early adulthood, but the vaccine was considered too expensive for the number of lives that would be saved for the NHS to fund it.

The Health Secretary, Wes Streeting, has asked independent experts in the Joint Committee on Vaccination and Immunisation (JCVI) to look again at the decision not to vaccinate teenagers.

There have been separate calls to consider an immunisation programme for university students because of their greater risk.

"If there is a change in the epidemiology and we start to see more cases and outbreaks as we recently saw in Canterbury that of course might change the balance of cost benefit and lead to a change in policy.

But we have to wait and see if that happens," says Prof Adam Finn, a professor of paediatrics at the University of Bristol and a former member of the JCVI.

"As things stand there is absolutely no reason for people to be more concerned about the risk of MenB disease than they were two weeks ago or two months before that – this remains an extremely rare if devastating condition," says Gil

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