They're called HIV "superinfections" and a study indicates they're much more common than first thought. Researchers say this raises concerns about possible resistance to treatment and may require new approaches to AIDS vaccine research.

Dr. Andrew Redd said there are HIV infections and then there are superinfections.

"A superinfection occurs when an individual is initially infected with a strain or strains of HIV. And then at some point later on – after that person has developed an initial immune response to their first infecting strain – at that later time point they come into contact through risky behavior with a second viral strain and then are superinfected with that second strain," he said.

Redd is the lead author of the study and a scientist at the National Institute of Allergy and Infectious Diseases. He said prior to the research done in Uganda it was thought that superinfections were rare, occurring in intravenous drug users or men who have sex with men. Not so.

"What we found in our study was that when we looked at a general population of heterosexual individuals in Uganda we found that it actually isn't as rare as what we thought. And that it is occurring at a significant rate even in the general population," he said.

So how does a person become superinfected? Well, it has to do in part with where you are and what HIV subtypes are prevalent.

"There's geographic separation between what we call the HIV subtypes, or clades. One of the interesting things about where we work in Uganda is it is at sort of the crossroads of two of those clades. The clades are labeled and named by letter – A, B, C, D. And in Uganda, we have subtype A and subtype D," said Redd.

Continued risky behavior after initial infection could lead to infection by another subtype of HIV. Current testing methods will confirm whether a person is HIV positive. But they cannot determine what HIV subtypes caused the infection. For that, the researchers need what's called "viral sequencing" -- an in-depth, complicated look at the viral DNA.

Left untreated, superinfection could lead to an accelerated progression of HIV/AIDS. Redd says, though, that standard antiretroviral treatment should be effective against superinfections.

"We have no evidence so far and we don't think that HIV superinfection affects the response to treatment. From what we can tell, individuals who get superinfected respond to treatment just fine and it lowers their viral load and they get healthier. One of the things that we're worried about, though, if the possibility that an individual could get superinfected with a viral strain that is already resistant to the antiretroviral drugs and that would be a major problem. But to date that doesn't seem to be a huge risk so far," said Redd.

Superinfections could also affect how HIV vaccine research is conducted. For example, there are individuals who are at risk of HIV superinfection but do not become superinfected. They are somehow protected.

He said, "One question would be what natural immune response to their initial infection is protecting them from the superinfection. And if we can figure that out that may give us a very interesting avenue to pursue for HIV vaccine research."

Redd added that current HIV vaccine strategies that try to "recreate the natural immune response may be insufficient to protect an individual from infection."

superinfection: 双重感染

Related stories:

US considers approving drug to prevent HIV

Campaign aims to help end childhood HIV infections

New studies on HIV treatment as prevention

Better treatment for HIV-positive pregnant women

（来源：VOA 编辑：旭燕）