What is the COVID-19 Delta variant?

Written by Dr David Owens

All viruses have a tendency to evolve over time. The SARS-COV-2 virus has already undergone many thousands of mutations, indeed almost every single infection in Hong Kong has its own 'genetic fingerprint'. This is one of the ways we are able to connect infections to each other. We follow how closely the ‘fingerprints’ match.

What is special about the Delta mutation?

In late 2020, researchers in the UK identified a new variant. This variant is now called the Alpha variant. Different variants were also recognised in South Africa (Beta variant), Brazil (Gamma variant) and India (Delta variant). Whilst the exact position of the mutations are different they share a number of characteristics including evidence of increasing transmission rates. That is to say, these variants are more infectious. They are responsible for the sudden and rapid increase in infections which we saw in Europe, South America and most recently in India.

The new variants have already spread worldwide. These variants have led to accelerations of the epidemic in the regions in which they seed. Imagine these variants as being like the iPhone. The iPhone 4 seemed like it was good at its job, but it evolved, the iPhone 5 was better and quickly became more popular. Often the evolution is gradual maybe the 5S but occasionally there is a leap, a larger screen, better camera and over a relatively short time that particular model becomes dominant. It’s the one that everybody wants and it tends to take over. Other companies mimic the success of the most popular brand. This is the way that viruses compete. Random mutations almost always give no benefit but occasionally we find a sudden jump and the virus becomes much better at travelling, fitter and faster, it spreads through the population more easily. The Delta virus is 60% more infectious than the Alpha, which itself nearly overran the US, UK and Europe 6 months ago. The Delta virus is spreading everywhere in the world.

We have previously explained the important difference between the severity of disease and the severity of an epidemic. COVID-19 has the potential to overwhelm health systems because of the rapid increase in numbers. The graph below demonstrates this issue. It compares a disease which is 50% more severe, continuing with slightly higher fatality rates, versus a disease which is 50% more infectious, producing a significant increase in mortality due to the rapid increase in the number of infected cases.

It is easy to lose sight of how well the epidemic has been managed in Hong Kong. COVID-19 is a mild disease in the majority of circumstances, but it has the potential to overwhelm health systems because of a potentially rapid increase in the numbers of infected patients. Avoiding excessive strain on the hospital system is one of the main goals of the public health controls. The new variants increase the risk of an epidemic taking hold and spreading quickly. We have seen recently in Taiwan how brittle a Zero Covid strategy can be.

There are only two ways in which an individual or population can develop immunity, either by contracting the disease or through vaccination. The potential impact of these more infectious variants spreading through our densely populated city makes widespread infection a serious health threat. If an epidemic took hold in Hong Kong we would expect to see high mortality rates and significant stress to the health system. Vaccination remains the best way of developing population immunity and protecting both our population and health system. This is the reason that public health doctors are advocating so hard for population vaccination even though the current numbers in Hong Kong are so low.

Do we need to increase quarantine again?

The short answer is no. There is no evidence to suggest that the variants of concern of COVID-19 have a longer incubation period. We can never keep this disease out of our city forever. We discussed the dilemma of when and how to transition from an elimination strategy in a previous article.

The Delta variant is spreading everywhere because it is the fittest and fastest variant. Evidence in the UK shows that 75% of infections are occurring in people who are not vaccinated but only 3.7% are occurring in people who are fully vaccinated. Cases in the UK are rising but fortunately, the vast majority of these infections are relatively mild and hospitalisations and deaths are not rising to the same degree as in previous waves. This is a direct result of vaccination, especially of the more vulnerable. The likeliest scenario, assuming we do not get a new mutation that escapes vaccination, is that the Delta wave will burn through the UK in the next 6 months and Covid will no longer be a threat as the population will hit Herd immunity by a combination of vaccination and infection. Europe and the US will follow 1-3 months behind.

It is important to appreciate that the Delta wave is not doing as much serious harm in the UK, Europe and US as previous waves because of the high levels of population immunity from previous infections and vaccinations. If Hong Kong had a significant epidemic we would expect to see significant strain to the health system and high mortality rates because we have low levels of immunity especially in our most vulnerable populations. It is natural and appropriate that the CHP is concerned about the potential for seeding of Delta variants into Hong Kong. The only solution is to develop population immunity through vaccination. Therefore there is a balance. Excessive public health controls and inconsistent communication have the potential to increase vaccine hesitancy and may well do more harm than good in the long term by reducing population immunity.

In summary, we already knew that some mutations of the SARS-COV-2 virus appeared to make it more infectious. As we have previously explained, COVID-19 is not a serious disease in the majority of circumstances but rather it is the nature of the epidemic and particularly the tendency to overwhelm health systems which causes harm. Fortunately, we have very good evidence that public health measures in Hong Kong, in particular test, trace, isolate in association with social distancing and masks are highly effective and there is no reason to believe that they will not continue to be so. The real challenge now is to overcome vaccine hesitancy and drive vaccination rates higher so that we can begin to return our lives to normal.