Genetically edited piglets are helping scientists take a major step on the path towards using live animal organs in humans, also known as xenotransplantation.

By removing potentially harmful retroviruses found in pig DNA, the experiments are opening up the possibility of humans one day safely receiving pig organs, according to the study published in the journal Science.

The first pig-to-human organ transplants could occur within two years, geneticist Dr George Church told The New York Times.

The scientists removed porcine endogenous retroviruses (PERV) in the pigs using the CRISPR gene-editing technique.

Researchers from Harvard University and biotech startup eGenesis took cells from pigs and snipped the viral DNA from their genomes, and then cloned the edited cells.

These edited cells were then developed into embryos and grown into genetically identical piglets.

The cloning process hasn't been fully perfected yet — out of the thousands of embryos transferred to 17 sows, only 15 piglets remain alive.

However they are all successfully PERV inactivated, with the oldest one at four months old. The scientists are now conducting long-term studies to see the impact of the gene editing.

The research was an "important advance" in addressing safety concerns about possible viral transmission during organ transplantation, eGenesis chief scientific officer Luhan Yang said.

There are still many other hurdles to overcome before successful xenotransplantation could occur.

The next stage of research would involve more genetic changes to pigs to modify the organs, as well as more lab tests before testing in humans can occur.

What's the deal with the retroviruses?

Since the 1960s, surgeons have made various attempts to transplant primate organs into people, although these were unsuccessful.

Scientists have been looking at the possibility of using pig organs in xenotransplantation for a while now — their organ size is similar to humans, they reproduce quickly and they can be genetically manipulated to reduce the risk of rejection.

Laika, at two days' old, was the first born PERV-inactivated pig. ( Supplied )

This was temporarily halted in 1998, when a group of researchers discovered concerns with PERVs in pigs.

They found human embryonic kidney cells became infected when researchers grew pig cells next to them.

These human cells then had the ability to infect other human cells, which raised concerns that transplanting the organs into humans could activate the virus and create a new human disease.

But it is still unclear whether PERVs can infect humans and several scientists — including Dr Jay Fishman, who helped author the 1998 study — have now become less concerned.

Pig heart valves have already been used in humans for several years, but are treated beforehand so they contain no living cells.

And the treatment of severe burns already involves animal tissue, with human skin cells cultured using animal feeder layers, harvested and then grafter onto the patient's wound to assist healing.

How many people (and pigs) could this affect?

In Australia, 1,713 people received organ donations in 2016.

And while the number of organ donors is rising year on year, about 1,400 people are waiting for organ donations at any one time, with an estimated 100 Australians dying on waiting lists each year.

About 4-5 million pigs are killed in Australia each year, mostly for meat.

But scientists argue the pigs involved in their experiments would represent a small percentage of that total, and would be used to save human lives.

In 2015, just under 1.3 million animals were used in research and teaching in Australia which caused at least a minor level of distress, according to Humane Research Australia.

The National Health and Medical Research Council (NHMRC) code for the care and use of animals for scientific purposes states that all teaching and research activities "must balance whether the potential effects on the wellbeing of the animals involved is justified by the potential benefits".