The evidence builds on previous studies on couples where one partner is HIV positive and on treatment, including a landmark study in 2011 that found that treatment can prevent new infections among couples by 96% and a second study in Europe in 2016 showing no transmission at all.
But scientists haven't given up hope.
"A safe and effective HIV vaccine would be a powerful tool to reduce new HIV infections worldwide and help bring about a durable end to the HIV/AIDS pandemic", said Anthony S. Fauci, Director at the National Institute of Allergy and Infectious Diseases (NIAID) in Maryland.
Dozens of potential HIV vaccines have been trialled down the years, but only one has passed phase three tests. And because of this, even after 35 years of research, still looking for a vaccine, effectively opposing him.
Among other things, that would mean that testing for it efficacy might be somewhat simpler, since participants could come from anywhere and be at risk of infection with any HIV clade.
While it may seem a big leap from cows to humans, scientists are encouraged by the results.
The ease of testing new vaccines will also allow scientists to tinker more and better understand how vaccines work. The immune responses varied in effect based on which boost vaccine was incorporated into the regimen.
In Swaziland, new HIV infections have been almost halved among adults, and HIV viral load suppression - a key marker of the body successfully controlling the virus - has doubled since 2011. Even after the virus goes away, the immune cells "remember" it and are able to produce antibodies quickly if a person is exposed to the virus again.
The trial in Thailand, led by the US Military HIV Research Program (MHRP), took people who were controlling their infection with their standard HIV medication. "The kind of insight we get from studying this is an understanding of the mechanisms whereby the cows' immune system is capable of creating these antibodies", Fauci told TIME. The two-year trial was conducted at 50 different medical facilities in Canada, France, Germany, Spain and the USA, and was funded by the injections manufacturer, ViiV Healthcare.
When the antibody binds to HIV in monkey studies, they form an "immune complex" which other parts of the immune system might be able to recognise.
Previously, the "Mississippi Baby", born with HIV in 2010, received anti-HIV treatment beginning 30 hours after birth, stopped therapy around 18 months of age, and controlled the virus without drugs for 27 months before it reappeared in her blood. One of these antibodies is particularly potent, NIH says.
The human body is inefficient at making antibodies that neutralise HIV. Also, these antibodies that longer loops of HCDR3 that develop faster.
The researchers injected the four cows with an HIV immunogen, a molecule that can prompt the HIV immune response.
"One of the great challenges for development of HIV vaccine is viral diversity", said Dan Barouch, a lead researcher on the vaccine and director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston.
Before now, when scientists tried to get immune cells in the lab to produce antibodies, the cells would do so indiscriminately, producing all sorts of antibodies, not just the relevant ones.
"We believe there may have been other factors in addition to early ART that contributed to HIV remission in this child", said Caroline Tiemessen, PhD, whose laboratory is studying the child's immune system.
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