A study from China traced a cluster of Omicron infections in Beijing to a contaminated letter received by post. However, experts advised caution and said airborne transmission is still the dominant route of infection for the virus
The yet-to-be peer-reviewed study by the Centre for Disease Control and Prevention (CDC) in Beijing noted that the variant of Omicron found in the package this January was genetically distinct from other cases in China but linked to those in Southeast Asia and North America.
The preprint is interesting but it does not make an adequately strong case for the indirect ‘surface’ mode of SARS-CoV-2 transmission, said immunologist Satyajit Rath, allaying fears of surface transmission of the virus.
“The gaps in the evidence, in the tracking and in the time periods involved are substantial. Therefore, I would be cautious about treating this preprint as the basis for making public health policies,” Rath, from Delhi’s National Institute of Immunology (NII), said.
Other experts agreed, saying surface transmission cannot be ruled out but airborne spread is the primary source of the spread of the infection.
According to virologist and genome sequencing expert Anurag Agrawal, “rare things” can always happen.
“However, SOP should continue to focus on airborne transmission of the SARS-CoV-2 virus,” Agrawal, director of the CSIR-Institute of Genomics and Integrative Biology in Delhi, said.
Rajneesh Bhardwaj, who has been studying Covid transmission with his colleague Amit Agrawal at the Indian Institute of Technology Bombay (IIT-B) since the beginning of the pandemic, added that airborne transmission is still a dominant route of Omicron infection in cases of community transmission in countries such as India and the US.
According to the CDC Beijing study, on January 15 this year, a 26-year-old female (Case A) in Haidian district, Beijing, tested positive for COVID-19 after complaining of excessive fatigue and fever for two days. The CDC subsequently verified the result.
A rapid site mutation test based on an RT-PCR test showed the case was infected by the Omicron variant.In the next seven days, five close contacts tested positive during screening, including her mother (Case B), a colleague of the index case (Case C), and three family members of Case C (Cases D, E, and F), the study said.
According to the authors of the China study, this is the first local cluster caused by the Omicron variant in Beijing.
“We proposed the infection was induced by the internationally mailed document… Field investigations showed no potential exposure of the case except the internationally mailed document, with an onset interval of two days,” the authors said.
They explained that all identified cases showed epidemiological links with Case A, adding that environmental surveillance identified SARS-CoV-2 RNA positive samples from the package’s contents, and part of these positive contents were untouched by the case.
More important, they said, the genome of the case matched the samples collected from the mailed documents, which differed from other local strains in China.
In Rath’s view, the report does not make its contribution hugely more prominent than what is currently thought — that only a minority of infections are caused by this kind of transmission.
“Indirect contaminated surface-based transmission of this kind has always been a possibility, and since the lineages of SARS-CoV-2 appear to survive a bit longer on some surfaces, it very much remains a possibility to be cautious about and to keep investigating,” Rath said.
Moreover, a study published in The Lancet journal in April last year found there is consistent, strong evidence to prove that the SARS-CoV-2 virus, behind the COVID-19 pandemic, is predominantly transmitted through the air.
The analysis by six experts from the UK, the US and Canada noted that public health measures that fail to treat the virus as predominantly airborne leave people unprotected and allow the virus to spread.
Experts also believe infected respiratory droplets, which are generated by coughing, sneezing and ‘moist’ speaking from an infected person, can land on a surface which can serve as a source of infection spread from a surface.
A study in 2004 showed that SARS coronavirus found on surfaces in a hospital and fomites should be considered to be a possible mode of transmission of SARS.Fomites are inanimate objects that, when contaminated with or exposed to infectious agents, can transfer disease to a new host.
“Fomite transmission is being recorded in China because of their zero-Covid approach. It shows that the fomite transmission is important once airborne transmission gets suppressed,” IIT-B professor Bhardwaj said.
China’s ‘zero-Covid’ approach has involved several measures like large-scale lockdowns, mass testing, international travel bans and strict vigilance on the virus. Once airborne transmission is controlled, focus can shift to surface spread of the disease, the experts explained.
“There is no clarity on the viable viral load (quantity) needed for the infection from a surface. However, there is a non-zero risk of the fomite transmission,” Bhardwaj said. His colleague Amit Agrawal added that the risk of the fomite transmission is not zero, as seen from the Chinese documented case of community transmission via fomite.
"The long survival times increases chances of the infection and coupling it with zero Covid policy by China, which suppresses airborne route, could explain the risks associated with fomite transmission,” Agrawal said.
In general, studies have shown that SARS-CoV-2 was less stable on paper than on plastic and stainless steel but Omicron survives on most surfaces much longer than the old strains.
“However, this fact does not rule out airborne transmission… The percentage of each transmission route in the total number of infections remains an open question,” Bhardwaj said.
The authors of the China study noted that temperature should not be neglected during the assessment for viral survival.
“The low temperatures in winter and in some countries located at high latitudes might lead to an extended viral survival period and should be considered as an essential factor in this cluster,” they said.
