Why has this meningitis outbreak spread so fast?

This meningitis outbreak is deeply unusual and defies easy explanation.

It has been described as unprecedented and explosive because there have been 20 cases since the weekend in one small area of Kent.

This is not the normal pattern.

Meningitis typically occurs as isolated one-off cases. It's now rare in the UK but occasionally there are small clusters, such as two infants at nursery in the north of England in 2023.

Bigger outbreaks have happened before. In the 1980s, there were 65 cases of MenB, including two deaths, in Gloucestershire but those cases were reported over four- and-a-half years not less than a week.

The burning question is: what's different this time?

How has an infection that requires close and prolonged physical contact, that spreads more slowly than measles, Covid or flu, caused such a rapid outbreak?

The answer is important, but not obvious – so far it appears to be an exceptional outbreak in seemingly unexceptional circumstances.

Even connections to the Club Chemistry nightclub – where 11 out of the first 15 affected had partied – do not give a complete picture. Students sharing vapes and drinks in a busy nightclub is a scene repeated up and down the country, rather than a unique event.

We know people regularly catch meningitis B bacteria and they usually live harmlessly in the nose. Across the UK about 10% of us have these bugs, but in teenagers and young adults it's as high as 25%.

It's only in a tiny number of cases that the bacteria cross the barriers inside our nose to invade the body and cause meningitis and sepsis.

For Prof Andrew Preston, from the University of Bath, there are two broad explanations for the numbers getting severely ill and dying in Kent.

He told me there has either been an "astonishing rate of transmission" meaning so many more people are catching the bacteria, or the infection is proving to be "more invasive" this time.

The underlying cause could reside in the bacteria itself, or in human behaviour, the environment or a combination of all of them.

Is this bacterial infection different?

Analysis so far shows the outbreak is being caused by group B meningococcal bacteria.

However, this is not a single entity – it encompasses over a hundred strains which all act differently in the human body. Some are more dangerous and more likely to cause invasive disease and meningitis.

Samples collected from patients are being analysed in the laboratory. So far it appears to be a strain that has been circulating for the past five years. Further analysis of the bacterial genetic code will reveal if it has mutated in a meaningful way. Further tests will investigate how the bacteria grows and behaves in the laboratory.

But there are other factors that can make it easier for meningitis bacteria to get from the nose into the body.

This is famously the case in the Meningitis Belt – which stretches across 26 countries in sub-Saharan Africa, from Senegal to Ethiopia. Dust, high temperatures and low humidity throughout the dry season are thought to damage the back of the throat and give the bacteria a route into the body. This triggers regular epidemics.

Smoking has been shown to have a similar effect and there is speculation about vaping in this outbreak. Sharing vapes among a group of friends, which is more popular than sharing a single cigarette, could be a route for meningitis to spread to a large number of people through saliva.

The act of vaping itself could irritate the airways and is known to cause inflammation, which some have argued could also make it easier for bacteria to get into the body.

But vaping is not a new behaviour or unique to Kent so does not, on its own, explain the exceptional nature of this outbreak.

The number of people needing hospital treatment at the same time suggests they were also infected at roughly the same time.

With at least 11 cases linked to Club Chemistry, the head of the UK Health Security Agency, Susan Hopkins, said: "This looks like a super spreader event with ongoing spread within the halls of residence in the universities."

Super-spreading events are moments when more people are infected than you would expect.

Meningitis bacteria do not transmit easily. They normally spread within households where people are in the same space for a long period of time. Nightclubs and university halls of residence are other mixing pots, which can give the bacteria the opportunity to spread, but again are not unique to this outbreak.

With other respiratory infections like Covid or flu, individuals who often have no symptoms, but have very high levels of the virus, can go on to unwittingly spread the infection to a large number of people. Whether something similar happened in Club Chemistry is unknown.

Prof Andrew Lee, from the University of Sheffield, suggests people with other infections that cause a lot of coughing and sneezing may have made it easier for meningitis-causing bacteria to spread in the club.

He said: "In the scientific literature there are some reported synergies between viral respiratory infections, for example flu, and meningococcal infections as the viral infections may potentiate the spread."

There are also questions about whether some people are born more vulnerable and at greater risk of severe outcomes. It is also possible that young people who spent their teenage years during Covid lockdowns may not have built up the usual amount of immunity to protect them against it.

"But that would be UK wide – so it may be one of the factors, but it can't be the sole explanation," says Preston.

There are still so many unknowns in this outbreak and we're still waiting for answers.

"I can't yet say where the initial infection came from, how it's got into this cohort, and why it's created such an explosive amount of infections," says Hopkins.