Velephi Okello of the Swazi health ministry told journalists at an HIV science conference in Paris.
The country -- where one in three adults is infected with the AIDS-causing virus -- has vastly expanded public programmes to test people for HIV infection and put them on life-saving anti-retroviral treatment (ART).
"The rate of new HIV infections has been reduced by half," Velephi Okello of the Swazi health ministry told journalists at an HIV science conference in Paris.
"Remarkable progress has been made... in controlling the HIV epidemic."
In 2011, 31 percent of adults (aged 18-49) out of a total country population of just over 1.2 million were infected with HIV, according to government data.
This made Swaziland the country with the highest national rate of new infections, said the authors of the new study, as well as the highest proportion of people living with HIV.
The latest data, based on blood samples from almost 11,000 people aged 15 and over, showed that about 27 percent of the population was HIV-positive in 2016.
This translated to an infection rate of 1.39 percent among 18- to 49-year-olds, down from 2.58 percent in 2011 -- a 46-percent reduction.
Ninety-five percent of HIV-positive pregnant women last year received drugs to prevent transmission of the virus to their offspring.
"As a result, fewer than 1,000 children became infected with HIV in Swaziland in 2016," said the UN agency.
The infection rate was higher among women than men, according to a survey report to the International AIDS Society conference. The decline was also steeper for men, with 52 percent, than for women (40 percent).
The survey showed that 73 percent of HIV-positive people had achieved suppression of the virus -- meaning it does not replicate to make them ill -- compared to 35 percent in 2011.
The gains were the fruit of a much improved HIV treatment programme, said the researchers. The share of infected people on ART rose from 37 percent in 2011 to 74 percent last year.
ART not only stops HIV from replicating and attacking a patient's immune system, but also curbs its spread to sexual partners.
"Our recipe for success is that we... have been able to scale up a lot of the prevention and treatment services in the country," said Okello.
"We have more than doubled the number of people who have started on anti-retroviral treatment, and we have also almost doubled the number of men who have been circumcised in the country."
According to the World Health Organization, there is "compelling evidence" that male circumcision reduces the risk of heterosexual sexual transmission of HIV by as much as 60 percent in men.
The number of HIV tests conducted in Swaziland more than doubled from 176,000 in 2011 to 367,000 in 2016.
"Basically, we think that that's one of the recipes (for success), and also the government commitment to buying and procuring the ARVs for people in the country so that there is a sustainable response going forward," said Okello.
Despite the "great news", she cautioned much more needs to be done to maintain the downward trend.
"While we do celebrate these findings, we still know that Swaziland is facing a severe HIV epidemic," she said.
"In the end, we would like to see a Swaziland which is free from AIDS."