Cheaper private Covid jabs could end up being just as expensive as their pricier alternative because the vaccine must be given in groups of five, experts have warned.

Boots and pharmacies that partner with the company Pharmadoctor are offering Pfizer/BioNTech jabs to those not eligible for a free vaccination through the NHS, with the former charging almost £100 a shot. The latter is also offering the latest Novavax jab, a protein-based vaccine, at a cost of about £50.

While experts have welcomed access to the jabs, they have also warned their high cost could widen inequalities.

Concerns have been raised over the use of the Novavax jab, given each vial of the vaccine contains enough for five doses, which – once opened – must be used within 12 hours if refrigerated, and preferably immediately.

Nathan Burley, a public health pharmacist and president of the Guild of Healthcare Pharmacists, said that while multidose vaccine vials were used during the pandemic for Covid-19 vaccines to maximise doses available for rapid rollout en masse, they come with a number of pharmaceutical concerns, including logistical issues.

“If patient scheduling is lax or patients do not attend then doses can be wasted,” he said.

Prof Lawrence Young, a virologist at the University of Warwick, said the Novavax jab was just as effective in protecting against severe disease as the Pfizer/BioNTech and Moderna jabs, and it was also cheaper and easier to store.

He said there was some evidence protein-based jabs had fewer side-effects, while other research had indicated a “mix and match” approach using different vaccines resulted in a better immune response.

He also stressed that once a Novavax vial was opened, five doses needed to be administered.

“This might not be cost-effective for pharmacies and could result in wastage of the vaccine if less than five people are booked in for vaccination,” Young said. “It could also result in pharmacies increasing the cost of the Novavax vaccine to compensate for any wastage.”

The Guardian has seen communications suggesting some pharmacies are already adopting a flexible charging approach.

One pharmacy sent out an email to customers stating Novavax jabs would be reserved for patients with multiple bookings involving friends or family. If only two individuals received Novavax jabs on the same day, the charge would be £90 per person.

“For three individuals, the charge will be £65 each, for four individuals, it will be £55 each, and for five individuals, there will be a flat rate of £50 per person,” the email stated.

Lara Wong, the founder of the support and advocacy group Clinically Vulnerable Families, said the approach was problematic.

“Coordinating group vaccinations to fully utilise each multidose vial presents a significant challenge, particularly for patients who, due to health vulnerabilities or logistical constraints, cannot easily organise themselves into groups. This could lead to higher individual costs for those unable to form groups. We are deeply concerned that this situation will introduce barriers to vaccination,” she said.

Graham Thoms, the chief executive of Pharmadoctor, said each of its partner pharmacies set its own prices, adding that Pharmadoctor did not charge a fee and did not take a cut from the pharmacy.

Thoms said strategies were in place to avoid wastage. He said: “Some of our individual partner pharmacists already received over 160 patient appointment requests from Pharmadoctor for April, therefore they will simply need to arrange the appointments in blocks of five patients so that there is no wastage. Most of our partner pharmacies are already planning for this.”