Since Weiss first published his results, much research has gone into the PERV problem. Good news seemed to come in 1999, when a study by Imutran and the CDC reported no signs of active infection in 160 patients who had received pig tissues. Obstacles on the road to human clinical trials seemed to be clearing. But, in late 2000 and early 2001, new reasons for caution emerged: During a transplantation of pig cells into mice, scientist Daniel Salomon recorded the first actual cross-species transmission of the PERV virus. At the same time, both the International Society of Heart and Lung Transplantation (IHLST) and the United Kingdom Xenotransplantation Interim Regulatory Authority (UKXIRA), issued reports stating that human clinical trials of whole organ xenotransplants were still "premature" and facing serious scientific hurdles. Even if scientists minimize the threat of PERV's, there is still the problem of hyperacute rejection of pig organs by recipients: To date, the longest survival rate of a pig heart in a non-human primate is 39 days.

Despite some of the setbacks, many researchers remain cautiously optimistic. As Salomon explains to FRONTLINE: " I think that we've made tremendous progress. I believe that there is nothing standing in our way that can't be solved by further scientific research. But we're not there yet." Here's a closer look at some of the recent reports:

In a 1999 report in the journal Science, Imutran, in collaboration with the CDC, announced the results of a study of 160 people who had received various pig tissues and/or cells. No active PERV infection was found in any of the patients, although 23 people were found to have active pig cells in their blood. However, since that study was published, more sensitive tests to diagnose the PERV virus have been developed, and this raised new questions for some. (According to UKXIRA's third annual report, "The findings from studies using these assays inevitably bring into question the reassurance offered by the Imutran/Novartis-CDC study.") Read commentary on the study in Scientific American and the Bulletin of the World Health Organization.

Furthering fears about the PERV virus, Daniel Salomon of the Scripps Research Institute reported the results of a study which found the first cross-species transmission of PERV during a transplant. He recorded transmission of the PERV virus while transplanting pig pancreatic cells into heavily immunosuppressed diabetic mice. The PERV cells lasted for as long as two months before going dormant. According to the report, "These results show that a concern for PERV infection risk associated with pig islet xenotransplantation in immunosuppressed human patient may be justified." In his interview with FRONTLINE, Salomon notes that although the results showed PERV infection, none of the mice became sick. The UKXIRA report concurs, noting that evidence that the virus exists in the system does not necessarily mean it will cause adverse effects in the recipient.

The International Society for Heart and Lung Transplantation, a nonprofit organization whose members include doctors, scientists, ethicists and nurses, contends that human clinical trials are still far in the future: "Current experimental results indicate that a clinical trial of heart xenotransplantation is premature. Experimental lung xenotransplantation is at an extremely primitive stage of development and no consideration for a trial can be given at the present time." The report advises that clinical xenotransplantation trials should not be undertaken until authorities have determined a minimal virus risk and 60% of pig organs survive in non-human primates for a minimum of three months. However, the report concludes on a reasonably optimistic note: "Xenotransplantation has the potential to solve the problem of donor organ supply, and therefore research in this field should be actively encouraged and supported."

In early 2001, UKXIRA, the regulatory body set up in 1997 to monitor xenotransplantation in the UK, concluded that whole organ clinical trials in humans "are clearly still some way off." However, UKXIRA has not written off the potential prospect of xenotransplantation. According to their report, "Although alternative therapies are in development, xenotransplantation may still offer the prospect of a viable treatment within a worthwhile time frame." In fact, UKXIRA faults overzealous researchers for inflated expectations about the promise of xeno organ transplants: "Public statements by researchers, that the initial hurdle of hyperacute rejection (HAR) had been largely overcome, led certain quarters of the media to anticipate a move to clinical trials in the near future."

While clinical trials for cellular xenotransplantation have been ongoing for more than a decade, the most recent results have not given cause for much optimism. In March 2001, Genzyme Corporation and Diacrin Inc. announced that in a clinical trial involving patients suffering from Parkinson's disease, the condition of those who received transplants of fetal pig cells did not improve any more than a control group who had the surgery, but were not implanted with cells. The study reported no detection of PERVs in the patients who had received pig cells. The companies have said that they intend to reassess the data before deciding whether to pursue further clinical trials.