A long-awaited report on a large and possibly still ongoing outbreak of MERS coronavirus in Saudi Arabia reveals the virus spreads easily within hospitals, at one point passing in a person-to-person chain that encompassed at least five generations of spread.

The study, co-written by Toronto SARS expert Dr. Allison McGeer, also hints there may have been a superspreader in this outbreak, with one person infecting at least seven others.

The study lays out what is known about an outbreak of MERS that erupted this spring in four hospitals in the Eastern Province of Saudi Arabia, in an area whose name translated into English can be spelled Al-Ahsa or Al-Hasa (the study uses the second version). It was reported online on Wednesday by the New England Journal of Medicine.

Superspreaders played a key role amplifying SARS cases during the 2003 outbreak. That, combined with the symptoms patients manifest when they become sick and the long and varied incubation period, paint a picture that is reminiscent of SARS for the authors, several of whom, like McGeer, worked in Toronto to contain that coronavirus.

"This virus is closer to SARS than anything else," McGeer, an infection control expert at Toronto's Mount Sinai Hospital, said in an interview from Cairo, where she was attending a World Health Organization meeting on MERS on Wednesday. She travelled to Al-Hasa to help investigate the outbreak in May and was on a WHO mission to Saudi Arabia earlier this month.

"If you want to think about how you're going to prevent and manage hospital outbreaks, SARS is the place that all of us would start."

The 2003 SARS outbreak infected more than 8,400 people in 31 countries, killing at least 916. In the main it was an outbreak of hospitals, wreaking its devastation on staff, patients and visitors of facilities in which it spread. MERS and SARS are members of the same viral family.

When is it contagious?

McGeer said there are some fortunate distinctions — few health-care workers appear to be getting infected with MERS — as well as some worrisome ones. The high attack rate among patients was "pretty unnerving," she said, as is the fact that it appears people are infectious earlier in their illness than SARS patients were.

SARS cases were contagious mainly late in their illness, which gave health authorities a chance to diagnose and isolate patients before they could make others sick. If MERS is contagious earlier in the course of illness "that's not good," McGeer said. "If it's true, it's a real challenge because identifying cases early with either SARS or MERS is really difficult."

Others too could see similarities to SARS based on the findings of the study.

"Clearly like with SARS the health-care environment is a significant risk factor for ongoing transmission," Michael Osterholm, an infectious diseases expert who has been following MERS closely, said after reading the paper. He was not involved in the work.

"It means that unless the index of suspicion is very high and patients are immediately handled with the highest level of infection control, you could surely expect that you're going to see additional episodes outside of the Middle East like are being seen in the Middle East."

Sustained spread

Osterholm, who is director of the Center for Infectious Diseases Research and Policy at the University of Minnesota, said when cases have arrived undiagnosed in European countries, hospitals there have been relatively quick to figure out what they have on their hands and take measures to protect against spread.

Still, imported cases triggered onward spread in Britain, Italy, Tunisia and France, where transmission took place in a hospital. In all those countries, transmission events ended after a generation or two of spread because of measures taken to isolate patients.

"The issue that we all worry about is what happens if this gets into a country that doesn't have the same level of infection control capability in a health-care setting and what does that mean?" noted Osterholm.

Saudi Arabia, which is one of four countries from which MERS has spread and which has recorded the lion's share of cases, draws millions of religious pilgrims each year from around the Muslim world. The kingdom is also host to many guest workers, people from poorer countries like the Philippines and Pakistan drawn to Saudi Arabia for work. Experts have worried both those patterns of movement could disseminate MERS to parts of the world that might be ill-equipped to battle it.

The study gives details of the first 23 cases in the Al-Hasa outbreak, infections that date from April 1 to May 23. It does not say whether the outbreak in Al-Hasa has ended, and the Saudi ministry of health has been silent on that matter.

The senior author of the New England Journal paper is Dr. Ziad Memish, the kingdom's deputy health minister. Memish has given few interviews on the topic of MERS and he did not reply to an email from The Canadian Press requesting an interview about the journal article and the Al-Hasa outbreak.

Broader testing and pneumonia

However, in the weeks since May 23, the kingdom's health ministry has announced 16 additional cases of MERS. The ministry statements, which offer the barest of details, suggest at least 11 of the new cases were from the Eastern Province.

The statements do not say if these new infections are part of the outbreak in the hospitals. Other possibilities are that they are people who caught the virus from its still-unknown source in nature or, potentially, from other infected people outside hospital settings.

McGeer said she does not know if any of the cases reported since May 23 are part of the Al-Hasa outbreak. She also does not know if the outbreak is over, but warned with a disease that has an incubation period as long as MERS — out to nearly 15 days in some cases — it can take a long time to be confident that transmission has been stopped.

She could not rule out the possibility that the chains of transmission in this outbreak might now stretch beyond five generations, saying only that it is possible they extend longer. That type of sustained spread suggests the virus has no difficulty going from person to person, at least in the right settings.

Dr. Trish Perl, another author who went with McGeer to Saudi Arabia, says this virus is capable of sustained spread.

"I think that's one of the more important things that we show, is that there really is ongoing human-to-human transmission. This is not a single generation," said Perl, an infection control expert at Johns Hopkins University in Baltimore, Md.

McGeer, Perl and their fellow authors acknowledged this outbreak may actually have been substantially larger than what it appears to be based on the confirmed cases. Nearly a dozen people were identified as probable cases, based on their exposure histories and their symptoms. They tested negative, but it has been seen elsewhere that some patients only test positive after repeated testing.

"I'm not sure that we effectively identified all of the probable cases outside of the hospital," McGeer said. "I don't think it was twice the size. But it was probably larger."

And some were not tested at all because the hospitals were using the WHO's case definition, which requires pneumonia. So people who were sick with respiratory symptoms and who had had contact with MERS cases but who didn't have pneumonia were not investigated as possible cases.

Perl said it may be time to amend the WHO definition, to encourage broader testing in situations like Al-Hasa.