Coronavirus: are two strains together deadlier than one?

Different strains of the new coronavirus could be infecting people in some communities at once, pushing up the death toll from the disease, according to a new study.
Researchers from the UC Berkeley school of public health said variations of the pathogen circulating in Europe and the United States could be causing “serial infections” in some people, confusing the immune system and triggering an overreaction or even death.
“If one strain is still highly prevalent, the situation should be closely monitored, especially for severe disease occurrence, and social distancing should still be maintained to make sure the second strain doesn’t get introduced,” Lee Riley, professor and chair of the division of infectious disease and vaccinology at the school and lead author of the study, told the South China Morning Post on Thursday.
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The team’s findings, which have not been peer-reviewed, were posted on the preprint service medRxiv.org on Wednesday.
Previous studies found that as Sars-CoV-2, the official name for the coronavirus that causes Covid-19, spread from person to person, it acquired an important mutation called D614G. This mutation could improve the physical stability of its spike protein and make it more infectious, according to some research.
The mutation is rare in China, accounting for 2 per cent of all the samples sequenced so far, including cases linked to overseas travellers. But by early this month, more than 70 per cent of the samples in the global database had this variation.
The D614G strain has further evolved into two major subgroups, one with one extra mutation (C14408T), and the other with two (C14408T, G2556T).
The former is now the dominant strain in western Europe, while the second is the most common in the United States.
At first, the researchers suspected these variations might pose different threats to people, but the theory was not supported by the data. They then looked at what happened if a region was hit at the same time by the strains dominant in Europe and the US, and found that the death rates tended to peak a few weeks after co-circulation ” even after adjusting for other mortality factors.
“(The finding) raises a disturbing possibility that people living in places with high prevalence of co-circulating strains may get serially infected with each variant,” the researchers said in the paper.
For example, Germany had low mortality at the early stage of the pandemic but the American strain arrived in March and quickly spread, accounting for as many as half of the cases at one point. Three to four weeks later, the death rates in Germany peaked.
The researchers found even if the second strain infected only a small proportion of the population, it could still cause a spike in deaths. In Britain, for instance, the American strain was present in only 6.4 per cent of its samples, but the actual number of people being infected serially could be high, and it helped push the British death rate to 14 per cent, one of the highest in the world, the researchers said.
The Californian city of San Francisco had a low death rate of 1.6 per cent, and it has been dominated by the American strain. But Santa Clara county in the same state suffered a co-circulation with the European strain and recorded a death rate three times higher.
Similar things could be happening in other parts of the world, but the picture remained unclear. “There are not many sequences deposited from Africa and South America now so it’s difficult to say what’s happening in those regions,” Riley said.
A Chinese government epidemiologist based in Shanghai said a large sample study would be required to prove that co-infection with the two or more mutated strains made a patient’s condition worse.
“But it is a very good theory. If true, the world is in a deeper trouble than we know,” the researcher said, asking not to be named because he was not authorised to speak to the media.
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