An “exceptionally” virulent variant of HIV has been detected in the Netherlands, where it has been circulating for several years, scientists say.

The variant, which is thought to have arisen in the late 1990s, has infected 109 people in the country, according to a research article published in Science.

Without treatment, the variant – referred to as VB – appears capable of accelerating a patient’s usual progression to advanced HIV and AIDS-related complications.

This is because it causes a decline in protective T cells that is twice as quick as other variants, the scientists said.

By the time of their diagnosis, the 109 individuals with VB were vulnerable to developing AIDS within two to three years, compared to six to seven years for people infected with other HIV variants, the research said.

Those infected with VB also developed higher viral loads at quicker rates, suggesting the variant may be more ineffective at transmitting than other variants.

However, Professor Jonathan Stoye, a virologist at the Francis Crick Institute, said the variant “doesn’t seem to have spread very widely or infected more people than we’d typically see with other variants”.

He added: “There are always trade-offs between the speed of virus replication and transmissibility. It has to be able to replicate to high levels to transmit, but it doesn’t necessarily mean it’s going to predominate.

“There aren’t any rules or we don’t know the rules that will tell us whether a given virus with a given set of properties will actually succeed in the evolution game. Whether it will grow and out-compete other variants, we don’t know.”

People infected with VB were present in all regions of the Netherlands, but with a different distribution relative to that of non-VB individuals, the paper said. VB individuals were more prevalent in the south and less common in Amsterdam.

Despite VB’s increased virulence, it remains vulnerable to current drugs, the researchers said. After receiving treatment, those infected with VB showed similar CD4 counts as people infected with other variants.

CD4, a type of T cell responsible for protecting the body against opportunistic infections, is targeted and attacked by HIV. Antiviral medicine helps to prevent HIV replication and restore the level of CD4 cells in an infected person, meaning they are no longer able to pass on the virus and can live a normal life.

“Had VB individuals not started treatment earlier than others, lower CD4 counts at treatment initiation would have been expected, potentially causing increased morbidity and mortality,” the researchers said.

Researchers assessed VB samples stretching back to one patient who was diagnosed with HIV in 1992. However, it wasn’t until 10 years later that the variant started appearing in other individuals.

Prof Stoye said it was unclear from the research why VB appears to be more effective at replicating than other HIV variants. “I’d like to know why all the changes that have occurred in this virus causes it to grow better, but there’s that there’s no clue to that question.”

There are more than 750 mutations in VB compared to other HIV variants in circulation in the Netherlands, the paper said.

Give the ability of VB to rapidly replicate in infected people and progress to advanced HIV, the variant is of “concern”, the scientists said.

“Without treatment, advanced HIV is expected to be reached in only nine months from the time of diagnosis for VB individuals, compared with 36 months for non-VB individuals, in males diagnosed at the age of 30 to 39 years,” they wrote.

“In contexts with less awareness and monitoring, in which diagnosis often occurs later in infection, the probability of reaching advanced HIV before diagnosis would be even greater.”

Prof Stoye said VB “might be more serious” in those countries where there is “less early diagnosis” and limited access to antiviral treatments, pointing to the example of Southern Africa. “But I don’t think we can predict whether it will come to in any way spread through these types of populations.”

Referencing coronavirus, Prof Stoye said that VB was proof that viruses don’t always evolve to become milder. “There’s been this widespread view that coronavirus will just get milder but I don’t think that’s true.

“Viruses can go up in terms of virulence, or down.”