Omicron is the name given to a mutant of SARS-CoV-2 first reported in South Africa. Subsequently, several other countries, including India, have reported cases of this variant.
Among those infected by this variant were a good many who had been vaccinated or had been infected by earlier Covid variants. Early data on the new variant show two trends: that there is a gap between infection and hospitalisation and that Omicron is likely to be less lethal than previous variants.
Several animal model studies have shown that Omicron lodges fairly readily in the upper airways but is not potent enough to directly infect lung tissue and that when it does, finds it difficult to multiply there. On the other hand, it has exhibited a capacity to spread fast among the population, both in South Africa and the United Kingdom, and compares with that of measles, the benchmark for high transmissibility.
It seems at the moment that although it spreads faster than its predecessor – Delta – Omicron is less dangerous. Several laboratory studies have emerged offering an explanation for the differences in behaviour.
Melanie Ott, a virologist at the Gladstone Institute of Virology in San Francisco, says that Omicron’s hyper-transmissibility means that hospitals are filling quickly. She adds, however, that the severity in patients is less.
South African health authorities have reported that the country passed its Omicron peak without a major spike in deaths. The UK Health Security Agency reported that Omicron-infected people in England were about half as likely to require hospitalisation or emergency care as were those infected with Delta.
However, there is still lack of clarity on how Omicron behaves in people immunised through vaccine or through infection, or both. Efforts are underway in laboratories, using animal models and cell cultures, to ascertain whether or not Omicron causes milder disease than earlier variants.
Studies by virologist Michael Diamond and colleagues on hamsters and mice conducted at Washington University in St. Louis, Missouri, have thrown up interesting results on disease progression. They found that the concentration of virus in the lungs of animals infected with Omicron was ten times lower, at the least, than that in rodents infected with other variants. Other teams have reported similar results. Michael Diamond has observed that Omicron-infected animals-maintained body weight, suggesting that the severity is manageable.
A team of researchers at the Cambridge Institute of Therapeutic Immunology and Infectious Disease, Cambridge, UK, found a protein called TMPRSS2 in lung cells that thwarts the entry of Omicron into the lungs. The team used a miniature lung model called “organoids”. Protein TMPRSS2 occurs on the surfaces of cells in the lungs and other organs but is absent from surfaces of most cells of the nose and throat cells, causing mild and casual infection there. The symptoms are similar to those caused in common cold.
The previous variants of Covid, however, have used TMPRSS2 protein to bind to cells, causing infection at the region of attachment. Dr. Ravindra Gupta, a virologist at the University of Cambridge, a member of the team engaged in TMPRSS2 studies, has expressed the opinion that Omicron infections are restricted to the upper respiratory tract.
There is also a suggestion that Omicron could have implications for children. Audrey John, paediatric infectious disease specialist at the Children’s Hospital of Philadelphia in Pennsylvania, says younger children have relatively small nasal passages – babies breathe only through their noses – and might suffer discomfort in the upper respiratory portion. However, the symptoms are restricted to those like in the common cold.
Melanie Ott agrees that there are mitigating aspects to the Omicron. “The virus establishes a very local infection in the upper airways and has less chance to go and wreak havoc in the lungs,” she says.
However, it has to be borne in view that ‘host- immune response’ plays a crucial role in disease severity and only when sufficient clinical data on the biological issues with respect to Omicron’s disease progression become available would a better explanation be possible. Even in the absence of detailed clinical data, trends as documented in South Africa and the UK have revealed that hospitalisation rates are not alarming and supports a qualified case for suggesting that Omicron is not as fearsome as its predecessors.
However, since viruses are continually evolving, departments of health care will have to continually evolve strategies for prevention and treatment that will include immunisation, hospitalisation and post treatment care protocols.
Dr. T.S. Channesh is with the Centre for Public Understanding of Science in Bengaluru