The number of cases of meningitis being investigated by health officials linked to Kent has risen to 20, up from 15 previously.

The UK Health Security Agency (UKHSA) said that, as of 5pm on Tuesday, 20 cases of meningitis had been reported, up from 15 on Tuesday.

Of these, nine cases have been confirmed in the lab and 11 remain under investigation. Six of the confirmed cases were found to be the meningitis B (MenB) strain.

A statement said: “All those affected who are currently linked to the outbreak are young adults. UKHSA is aware of a baby with confirmed meningococcal group B infection who is not currently linked to the outbreak but UKHSA will continue to investigate this case.”

Trish Mannes, the UKHSA regional deputy director for the south-east, said: “Around 2,500 doses of antibiotics have now been administered across sites in Kent and we continue to encourage close contacts to come forward for the treatment.

“This includes those offered at the University of Kent and anyone who visited Club Chemistry between March 5-7. This is the main intervention that will help protect people and halt the spread of the outbreak.

“As a further precaution and together with the NHS, we are beginning to roll out a targeted MenB vaccination programme. This will initially be offered to 5,000 university of Kent students resident at the Canterbury campus, with the possibility that it may be extended, as it is kept under continual review.

“If you think you may have symptoms of meningitis, don’t hesitate to seek medical help by contacting your GP or calling NHS 111. Seeking early treatment can save lives.”

However, Olivier Picard, the chair of the National Pharmacy Association, told BBC Radio 4’s Today programme there was currently no stock of MenB vaccines for people to pay for privately.

He said pharmacies were used to “bridging the gap between what the NHS offers and what patients want”, but added: “Unfortunately, that supply has run out, and most of our distributors, wholesalers have no stock. And whilst we’re hearing that there may be some stock in the system, it is taking its time to come into our fridges. There’s no date of resupply.”

The outbreak is being viewed by experts as unprecedented owing to the high number of cases appearing in a short time.

The UKHSA chief executive, Susan Hopkins, said: “This looks like a super-spreader event, with ongoing spread within the halls of residence in the universities. There will have been some parties particularly around this, so there will have been lots of social mixing.

“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. I can say that in my 35 years working in medicine, in healthcare and hospitals, this is the most cases I’ve seen in a single weekend with this type of infection.

“It’s the explosive nature that is unprecedented here – the number of cases in such a short space of time. NHS were initially managing it as a major incident in the region but they have now increased that overlay to having a national-level oversight as well.”

England’s deputy chief medical officer, Dr Thomas Waite, said: “This is by far the quickest-growing outbreak I’ve ever seen in my career, and I think probably any of us have seen, of meningitis for a very long time.

“Whilst it remains an outbreak that is having its consequences in Kent, it is obviously of national significance.”

About 5,000 students in university halls in Kent are to be offered the MenB vaccine in coming days. The aim is to prevent further disease in several weeks’ time if somebody has been harbouring the infection.

Four schools across Kent have now confirmed cases and hundreds of people are being offered antibiotics as an immediate treatment.

On Tuesday, the health secretary, Wes Streeting, said most students would not already be vaccinated against MenB, adding that the jabs will begin in the next few days. He told MPs: “This is an unprecedented outbreak. It is also a rapidly developing situation.”

Streeting said the MenB vaccine had been available on the NHS since 2015 as part of routine childhood immunisations, “but clearly most students would not be vaccinated”.

He added: “Given the severity of the situation, I can confirm to the House that we will begin a targeted vaccination programme for students living in halls of residence at the University of Kent in Canterbury, which will begin in the coming days.”

The programme may also expand further if other groups are deemed to be at risk.