The United Kingdom is inviting its most vulnerable residents for follow-up booster jabs, after a spike in COVID-19 infections was partially attributed to waning immunity to the novel coronavirus.

The follow-up booster jabs will be offered, starting this week, to around 5 million of the nation's 68 million inhabitants, including those aged 75 and older, care home residents, and people deemed clinically extremely vulnerable because of other medical conditions.

The Joint Committee on Vaccination and Immunization, an independent UK organization that advises health departments on inoculation issues, said the vulnerable groups were being invited for an additional jab as a "precautionary" move and not because there was evidence of imminent danger.

Amanda Pritchard, National Health Service England's chief executive, said steps to protect people from COVID-19 would continue for some time.

"We have protected millions of people at speed, thanks to the efforts of our staff, aided by volunteers," she said. "They will once again rise to the next challenge."

Pritchard said the NHS will contact people who are eligible for a second booster jab. Individuals do not need to contact health agencies proactively.

Official estimates suggest around one UK person in 20 currently has COVID-19, and that prevalence is continuing to rise.

Experts say waning immunity caused by the efficacy of vaccines starting to fade six months after they are given, the emergence of more-transmissible strains, and the end of the UK's restrictions on movement and socialization have all played a part in allowing cases to quickly multiply.

Health Secretary Sajid Javid urged people "to come forward as soon as you are contacted by the NHS", so they will continue to be protected against serious illness and death.

"Our phenomenal vaccination program has saved countless lives and built a wall of defense which has allowed us to learn to live with COVID," he said.

The UK Health Security Agency, which is a government agency responsible for UK-wide public health protection, said 157,000 hospitalizations have been prevented since December because of the nation's booster-jab program.

Maggie Throup, the UK's vaccines minister, said: "We must ensure those at greatest risk of serious illness from COVID are protected, and spring boosters will top up people's immunity."

The UK administered its first novel coronavirus immunization in December 2020 and has since given 118 million vaccinations, including 32 million top-up jabs delivered through the booster-jab program.

The UK's recent spike in infections has been particularly pronounced among people aged 75 and older, with infections hitting a record high during the second week of March, due largely to the emergence of the highly infectious Omicron BA.2 subvariant, the Office for National Statistics reported.

Some 3.3 million people had COVID-19 during the week ending March 12, some 26 percent more than during the previous week. And, as of March 17, 14,671 people were in UK hospitals with COVID-19; a rise of 38 percent on the total from two weeks earlier.