Doctors in London believe they could be on course to defeat HIV after a dramatic fall in the number of new infections.

An early intervention program imported from San Francisco has seen a 42 per cent reduction in new cases of HIV at 56 Dean Street in Soho, the biggest sexual health clinic in Europe.

The clinic uses a RITA blood test to spot patients who have contracted HIV within the past four months — when they are “super infectious”.

The Recent Infection Testing Algorithm (RITA) is a generic name for a number of laboratory techniques which distinguish recent and established HIV infection. They may also be known as incidence tests or as STARHS (Serological Testing Algorithm for Recent HIV Seroconversion).

RITA approaches depend on looking for specific antibody markers, which give different results in the months following infection. If a test gives a result below a pre-determined cut-off point, it is deemed to be a recent infection. The definition of ‘recent infection’ depends on the test used, but may be four to six months.

Patients are being diagnosed within several months of acquiring HIV and receiving antiretroviral drugs a week later, dramatically reducing their infectiousness and limiting the spread of the virus to other sexual partners.

Similar reductions in new cases were also achieved last year at other London HIV clinics such as Mortimer Street, Homerton hospital and St Bartholomew’s hospital, the Standard was told.

Dean Street saw the number of HIV diagnoses fall from 679 in 2015 to 393 last year — while continuing to perform “tens of thousands” of HIV tests.

Dr Alan McOwan, lead clinician at 56 Dean Street, said: “I have been working in sexual health for 20 years. It [the number of new HIV infections] has always gone up or stayed stable.

“This [drop] is what I have been waiting 20 years to see. Everyone is so excited. It makes people feel it’s achievable and we can finally beat this thing. It’s London’s time to grab its chance.”

The number of HIV diagnoses in London was broadly unchanged from 2014 to 2015, falling slightly from 2,671 to 2,603 people, including 1,373 so-called “men who have sex with men”. The capital accounts for 43 per cent of new HIV cases in the UK.

Public Health England estimates that one in seven gay and bisexual men in London are living with HIV, compared with one in 25 nationwide.

The findings from the clinic, which is run by Chelsea and Westminster Hospital NHS Foundation Trust, are significant as it typically accounts for half of all London diagnoses in gay men and one in nine of all HIV cases in the UK.

Advances in treatment has transformed HIV from a fatal infection into a “chronic manageable condition”, according to Public Health England, but there were 594 deaths of people with HIV last year in the UK.

Dr McOwan believes the drop in infections in London has exceeded that in San Francisco because the National Health Service allows free access to healthcare. In the US, delays in receiving treatment are caused by the need to have medical insurance.

Dr McOwan said: “We were determined that 2016 would be the year that London learned from San Francisco’s success. This drop in new HIV diagnoses, if confirmed, would be really significant as the clinic is a major contributor to HIV diagnosis in the UK.

“Reports from other London clinics suggest this could be regional thanks to better HIV awareness, frequent testing, early treatment and use of prevention methods such as PEP and PrEP in key populations.

“We’ve also concentrated on offering treatment to reduce infectivity to others. In the last six months, 76 per cent of our clients started HIV treatment at their first appointment.”

San Francisco has paved the way in Aids and HIV initiatives since the anti-prejudice work of campaigner and politician Harvey Milk — played by Sean Penn in the 2008 Oscar-winning film Milk — in the Seventies.

The Getting to Zero initiative in San Francisco seeks to achieve zero new infections, zero deaths and zero stigma for people with HIV by 2020.

“I think what San Francisco has shown is that you win against HIV by throwing everything at it,” Dr McOwan said.

In addition to antiretroviral drugs, Dean Street urges its clients to use condoms and helps high-risk groups who have unprotected sex to obtain the PReP drug Truvada cheaply to reduce their risk of acquiring HIV