The American drug giant said there was only a small drop in potency against the highly transmissible variant compared to earlier versions of the virus.
The findings - in line with how other vaccines have fared against the mutant strain - will reassure the more than 12.4 million Americans who have been jabbed with J&J's shot.
There had been concerns that the rapid rise of the Delta variant in the US - where it makes up 26.1 percent of cases - could derail the country's hugely successful vaccine rollout.
But the fact that the UK has managed to keep deaths and hospital rates low, despite the variant accounting for 99 percent of cases in the country, has given confidence that the crisis can be kept under control in America.
It also means that the majority of the world's approved coronavirus vaccines have now been shown to be highly effective at preventing serious illness from the strain.
The findings were detailed in a press release published by the New Brunswick, New Jersey-based firm on Thursday night.
Experts took blood samples from eight volunteers who participated in early-stage trials of the vaccine.
The samples were then exposed to the Delta variant and tested for antibody levels.
They found that levels of neutralizing antibodies only fell by 1.6-fold when tested against the Delta variant compared to the original strain.
'Today's newly announced studies reinforce the ability of the Johnson & Johnson Covid-19 vaccine to help protect the health of people globally,' said J&J Chief Scientific Officer Dr Paul Stoffels in a statement.
'We believe that our vaccine offers durable protection against COVID-19 and elicits neutralizing activity against the Delta variant.'
'This adds to the robust body of clinical data supporting our single-shot vaccine's ability to protect against multiple variants of concern.'
In fact, the study found that neutralizing antibodies level fell more steeply against other variants, including 3.6-fold against the Beta variant, which originated in South Africa, and 3.4-fold against the Gamma variant, which originated in Brazil.
The levels were still high enough to be effective, but encouraging because these variants are far less widespread in the U.S.
What's more, a sub-study conducted by Beth Israel Deaconess Medical Center looked at 20 volunteers and found that immune responses were still strong after eight months.
This included T-cells generated by the vaccine such as CD8+ T-cells, which seek out and destroy virus-infected cells.
People had more of these antibodies eight months after being jabbed than they did 28 days after having the vaccine, it found.
'Current data for the eight months studied so far show that the single-shot Johnson & Johnson COVID-19 vaccine generates a strong neutralizing antibody response that does not wane' said Dr Mathai Mammen, global head of research and development at J&J's Janssen vaccine arm, in a statement.
'Rather, we observe an improvement over time. In addition, we observe a persistent and particularly robust, durable cellular immune response.'
Covid is now more like a 'bad cold' thanks to the effect of vaccines, a top epidemiologist has claimed after data showed symptoms of the disease are becoming milder across the board despite rising cases.
King's College London's Covid symptom study estimated there were 25,210 new cases every day in the UK last week, up by almost a third (31 per cent) from the previous seven-day spell.
It said there was a 50 per cent increase in the number of partially or fully vaccinated people catching the virus — but in most cases their symptoms were mild. More than 80 per cent of infections were among the unvaccinated.
Professor Tim Spector, who leads Britain's biggest Covid surveillance study, said people catching the virus after being vaccinated suffered a milder form of the disease similar to a cold, with sneezing emerging as a new symptom.
'While rates of Covid infection are high, it's reassuring to see vaccinations protecting the vulnerable and deaths remain very low,' he said.
ZOE Covid study data shows symptoms are more mild and are similar to those of a bad cold, with a runny nose, headache and a sore throat among the top symptoms for all groups. Sneezing has also emerged as a symptom among partially and fully vaccinated people.'
An earlier trial by J&J found that a single dose of its vaccine was 85 percent effective at preventing severe disease from Covid. It also reduces the risk of being hospitalized or dying from the virus among all age groups.
However, J&J stressed that its vaccine provides protection against all if the so-called 'variants of concern'.
In the U.S., 200 million shots have been ordered, but only around 21.4 million have been delivered to states and 12.4 million have been administered.
The company has been plagued by several setback, including contamination problems at a Baltimore factory that helps manufacture the shot and pause after links to rare blood clots.
In April. the Johnson & Johnson vaccine was paused for 10 days after six women under the age of 50 developed CVST,.
This figure was later updated to include 28 people, including one 45-year-old woman who died.
J&J doses are expected to arrive in the UK sometime this autumn after getting approval from the medicines regulator in late May.
The vaccine can be given to people aged 18 and over and is likely to be used as a booster jab for care home residents ahead of winter because it can be easily stored and transported at fridge temperatures.
The UK's Joint Committee on Vaccination and Immunisation (JCVI) will issue advice on exactly who should receive the Belgian-made jab before its officially deployed.
There is a chance it could be restricted in younger age groups because of its link to extremely rare blood clots.
Like AstraZeneca's Covid vaccine, which is not being used in under-40s in Britain for the same reason, the J&J shot uses adenovirus technology to stimulate the immune system.
This appears to trigger a devastating immune overreaction in a tiny number of people, mostly young and healthy.
Trials have shown the vaccine - which US regulators approved in February - to be 67 per cent effective at blocking Covid symptoms from earlier forms of Covid. Other studies have shown it is even better at preventing patients falling severely ill.
No10's Vaccine Taskforce originally secured 30million doses of the vaccine last year, based on predicted demand at the time.
But because of the huge success of UK's vaccination program, ministers reduced their order to 20m.
In April, the European Medicines Agency ruled that the jab should come with a clear warning about a serious blood clotting disorder. It made the same recommendation for AstraZeneca's jab.
In April, the firm itself asked Europe to pause the roll-out of the jab to allow experts to probe the clot cases thoroughly. It later concluded the risk was rare and urged all countries to keep using it.
Officials insist the disorder - the same as the one seen in AstraZeneca's vaccine - is extremely rare but seems to be happening slightly more often in young people who have been vaccinated.
https://www.dailymail.co.uk/news/article-9748533/Johnson-Johnson-vaccine-effective-against-against-Indian-Delta-variant.html
