The United Kingdom health regulator has approved use of the COVID-19 vaccine from United States pharmaceutical company Johnson& Johnson, and 20 million of the jabs are expected to help the nation's booster campaign later this year.

The treatment, which is manufactured by Janssen, a Belgium-based subsidiary of Johnson& Johnson, is the fourth vaccine to gain approval in the UK after shots from AstraZeneca, Pfizer and Moderna.

"This is encouraging news for the public and the healthcare sector," June Raine, chief executive of the UK Medicines and Healthcare products Regulatory Agency, or MHRA, said of the authorization, which is limited to over 18-year-olds. "We now have four safe and effective vaccines approved to help protect us from COVID-19."

The Johnson & Johnson vaccine has some notable advantages over other jabs available in the UK. Whereas other treatments require two doses administered several weeks apart, the Johnson & Johnson jab is single dose. It can also be stored at routine refrigeration temperatures.

"As Janssen is a single-dose vaccine, it will play an important role in the months to come as we redouble our efforts to encourage everyone to get their jabs and potentially begin a booster program later this year," said UK Health Secretary Matt Hancock.

The Johnson& Johnson jab could be used as a booster shot ahead of the winter, especially in care homes and among vulnerable populations. The vaccine is currently part of a study that is exploring third doses that might complement vaccines already in circulation and provide possible protection against COVID-19 variants, as well as renew fading protection among people who received treatment many months ago.

"The Janssen vaccine will be another weapon in our arsenal to beat this pandemic," said Nadhim Zahawi, who is UK vaccines minister.

Rollout of the Johnson & Johnson jab had earlier been delayed in the US and parts of Europe due to concern over vaccine-induced thrombosis and thrombocytopenia, or VITT, which is a type of rare blood clotting.

A link between rare blood clots and the AstraZeneca vaccine is thought to be more probable, especially in certain age groups, and the MHRA concluded that people under 40 years old should not be offered that vaccine.

Separately, UK health authorities could not establish a firm relationship between the Johnson & Johnson jab and clotting.

"VITT cases have also been reported following vaccinations with the Johnson& Johnson/Janssen vaccine, a vaccine deploying the same technology as the University of Oxford/AstraZeneca one," said Cristiana Vagnoni, a biological sciences and health adviser for the UK Parliamentary Office of Science and Technology. "However, there is currently no evidence to show that it is the technology that is triggering VITT."