The World Health Organization (WHO) this afternoon reported four new laboratory-confirmed cases of a novel coronavirus infection, bringing the total number of cases identified since June to six. Two of the people infected are from the same household, raising the possibility of human-to-human transmission of the virus, although it’s also possible that they both contracted it independently from an animal source in the area. Three of the new cases occurred in Saudi Arabia (where one patient died) and a fourth case was reported in Qatar. The WHO gave few further details of the cases, such as their age, sex or current medical condition.

That the new cases have been found probably reflects increased surveillance for the virus since its identification as a novel virus. They follow two cases reported earlier — a 60-year-old man from Saudi Arabia who fell ill and died in June, and a 49-year-old man from Qatar who fell ill in September and has since recovered (see ‘SARS veterans tackle coronavirus‘). None of the contacts of those two men are known to have contracted the virus, which suggests that the virus probably doesn’t transmit, or doesn’t transmit easily, between humans. The novel coronavirus, identified as such in September, is genetically most closely related to bat coronaviruses, and bats (and possibly intermediate animal hosts) are likely to be the virus’s reservoirs.

Among this new batch of cases, two of the four people infected in Saudi Arabia were from the same family, living in the same household — one died and the other recovered. Moreover, two other family members showed similar symptoms, and one has died; the WHO is waiting to find out whether the fatal case tests positive for the coronavirus too. The recovered patient tested negative. Household clusters of cases of a novel virus raise the possibility of human-to-human transmission, and so immediately catch the attention of epidemiologists, who, along with clinicians and virologists, will be urgently seeking to tease out the probable source of infection, how the people contracted the virus and whether they each caught it independently from an animal reservoir or there is any human-to-human transmission going on.

The WHO gave no description of the symptoms, but the cases in June and September had severe pneumonia and acute renal failure. What’s also striking so far is the very high case mortality rate — so far two out of six cases, and three out of seven if the unconfirmed fatal case tests positive. Now, one can’t put a firm figure on case mortality rates until one knows the true number of deaths and cases, in particular as more asymptomatic or recovered cases may be going unnoticed — pinning that down will require more surveillance and epidemiology data, including seroprevalence surveys to test whether people not showing infection have antibodies to the virus (meaning they have been exposed to it). But the symptoms are serious, and the death rate in this particular cluster reinforces the impression that the mortality rate is high.

Not surprisingly, WHO is urging all countries to continue careful surveillance for severe acute respiratory infections, and warns that until more is known, it would be “prudent to consider that the virus is likely more widely distributed than just the two countries which have identified cases”. The WHO also seems to have upped the urgency of screening. Its current novel coronavirus case definition only calls for testing any unexplained pneumonias in places “where infection with novel coronavirus has recently been reported or where transmission could have occurred”. Today’s report says that testing of such cases for the novel coronavirus should be considered “even in the absence of travel or other associations with the Middle East”, adding that any clusters of serious pneumonia, or cases of such illness in health-care workers, “should be thoroughly investigated regardless of where in the world they occur”. For the moment, there’s no evidence that the virus transmits between people, but scientists and public-health officials will be keeping a close eye on it and seeking to quickly understand every aspect of the virus and its ecology.