Once again we have seen a new variant of the Covid virus emerge, and health officials are warily eyeing the data from around the world. With Omicron, a key question is whether infections are as severe as those caused by previous variants. Many are speculating that it could be inherently milder, especially after several preliminary reports from South Africa seemed to suggest fewer hospitalisations and less severe symptoms.
In my opinion this judgment is premature. Especially in such a rapidly changing situation.
Accurately estimating the severity of a new infection is a nightmare. Especially at such an early stage. However, there are some things we should keep in mind when evaluating the numbers coming in.
Looking at the severity of infections in South Africa, it is important to remember differences in populations. South Africa has a younger population overall, and Covid in younger age groups has been consistently milder than in older ones. There is also the issue of time, and how long it takes for people to fall ill and get sick enough to require care, and for those numbers to be reported. We are still only a few weeks into this newest wave.
Similarly, we do expect infections in a highly vaccinated population such as that of the UK to be milder overall, because vaccination is expected to protect against severe illness. However, that is little comfort to those who have been unvaccinated for various reasons, or otherwise unable to mount a robust immune response after vaccination. The surge of Omicron makes it more likely such people will be exposed.
Indeed, because Omicron appears to spread even more quickly than previous variants, we need to be cautious even if breakthrough infections are only occasionally severe, because there are going to be so many breakthrough infections. The ultimate number of severe outcomes is determined by the number of opportunities the virus gets to cause them, which is the result of infectiousness. This is one reason why the Alpha variant wreaked such havoc last winter.
You cannot simply compare Omicron with Delta by adding up the numbers of cases and hospitalisations for each over a short timeframe. The numbers of Omicron cases have been growing faster – much faster – than those of Delta, and it takes time for people to become seriously ill. To accurately gauge the severity, we will need to compare the numbers of cases that end up in hospitalisation or death. Mortality is often assessed as death within 28 days of diagnosis, and Omicron was reported to the WHO less than 28 days ago. We cannot afford to wait to find out exactly how dangerous it is.
There is a widespread belief that infectious diseases evolve to become less virulent, leaving many hopeful that Omicron will be less severe for everyone, regardless of age or vaccination status. This is false. Viruses do not necessarily get selected to be milder or more severe. If virulence (the severity of the disease) is not connected to transmission (the factor that makes a virus successful or not) there’s no real link between the two in most real situations. The great majority of Covid transmission occurs before people become seriously ill, and so the virus has already moved on.
We should remember that Delta was actually more virulent than Alpha, which itself was more virulent than the original virus. Remember the issue with Covid has never been its virulence. It’s the number of people infected.
Everyone wants to believe Omicron is milder. I would like it to be milder myself, but I am very careful to not allow what I want to colour the way I interpret the data.
The growth rate of Omicron is such that even if it is milder in most cases, cases can still rapidly add up and threaten the NHS. The UK has a healthcare system already struggling with decades of underinvestment and which was teetering on the brink after months of Delta. People seem to forget that nearly 20,000 people have already died from Covid in the UK since “freedom day” in July. The virus has been much more manageable, but that attrition has not been without consequence.
The most definitive estimate of severity that we can expect will come in a few weeks when the UK Health Security Agency (which has done a phenomenal job in terms of analysis in a dizzying turnaround time) reports on the comparison of the outcomes of infections with Delta and Omicron over the same time period. Before then, the South African data should start to become informative on longer-term outcomes. Very early data suggest hospitalisations in Guateng have peaked at a lower level than previous waves. But this is the first wave to have taken place in the presence of immunity of previous infection, which makes things more complicated to interpret.
Epidemics of rapidly transmitting viruses are not kind to overstretched healthcare systems, especially ones that have been fighting a pandemic bear for nearly two years already. Boosters will help, but Omicron will be infecting a lot of people receiving boosters now before the doses have had time to act to prepare the immune system. This is why Jeremy Farrar of the Wellcome Trust was right, even before Omicron, to propose “vaccines plus” rather than relying solely on the jabs to preserve healthcare.
Even if, and it is an exceedingly big if, Omicron is milder than Delta, that is no reason to relax. The transmission properties alone make this serious. The difference between a caress and a slap is largely in the pace with which it is given.
- Dr William Hanage is a professor of the evolution and epidemiology of infectious disease at Harvard and the co-director of the Center for Communicable Disease Dynamics
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