A vaccine from Johnson & Johnson is 66% effective at preventing moderate to severe Covid-19 but offers high protection against people needing to go to hospital, trial results show.

The single-shot vaccine, which has been developed by Johnson & Johnson’s pharmaceutical arm Janssen, is 66% effective overall at preventing moderate to severe Covid-19 28 days after vaccination.

The firm said the jab was 85% effective in preventing severe disease “and demonstrated complete protection against Covid-19-related hospitalisation and death as of day 28”.

The jab worked across multiple variants of coronavirus, including the South African variant which has been worrying scientists, the firm said.

The UK has ordered 30 million doses of the vaccine, with the option of millions more.

– What do the results show?

The vaccine is 66% effective at preventing moderate to severe Covid-19 but offers high protection against people needing to go to hospital.

Johnson & Johnson said the jab was 85% effective in preventing severe disease “and demonstrated complete protection against Covid-19 related hospitalisation and death as of day 28”.

– What else is exciting?

A major advantage of the J&J vaccine is that it can be given as a single dose.

This means it can be rolled out across populations much more quickly than two-dose vaccines.

Another major advantage is that it does not require ultra-cold storage like the jabs from Moderna and Pfizer. It can be kept at fridge temperature for at least three months.

The UK has struck a deal for 30 million doses of the vaccine, with the option of ordering 22 million more.

– How does the vaccine work?

Unlike the mRNA vaccines from Moderna and Pfizer, the J&J vaccine uses an adenovirus – a type of virus that causes the common cold – which has been modified so it cannot replicate and cause illness.

This is very similar to the approach used by Oxford University and AstraZeneca, which have also used an adenovirus.

We already know that the Covid-19 virus is studded with spike proteins that it uses to enter human cells – these spike proteins are the target for vaccines.

The J&J vaccine is based on the virus’s genetic instructions for building the spike protein and uses double-stranded DNA.

The gene for the coronavirus spike protein has been added to the adenovirus. Following injection, this genetic material is read by the body’s cells to mount an immune response to the perceived threat.

If, later on, the vaccinated person comes into contact with the virus, the immune system is prepared to attack it.

Antibodies and immune cells work together to kill the virus, prevent its entry into the body’s cells and destroy any cells that are infected.

– Has this technology been used before?

Yes, J&J has used this technology before in an Ebola vaccine and in investigational vaccines for HIV, both of which have accrued long-term safety data.

Experts believe the immune system contains special cells called memory B cells and memory T cells which might retain information about the coronavirus for a long time.

Scientists in the UK expect coronavirus vaccines to become an annual jab, with yearly modifications to adapt to different strains of the virus.