Delta variant: is it serious?

You bet. Health authorities and experts keep reminding us that delta variant represents the ‘greatest threat’ to the nation’s effort of eliminating Covid-19. Some experts even warned us that the Delta variant is deadlier than the original Alpha variant. I have done a little research and found that some of the experts’ assertions seem unfounded.

We all know that the Delta variant was first identify in Maharashtra (India) in October 2020. Since then, the variant has been found in more than 77 countries or territories in the world [1]. In the United States alone, Dr. Anthony Fauci stated that the variant accounted for 20% of total Covid-19 cases. In the UK, it was reported that over the past 28 days there have been 38,000 cases of Delta variant (BBC 20/6/21), making it the dominant strain of the coronavirus over there.

We are told that the Delta variant is highly contagious. The New South Wales Premier, Ms Gladys Berejiklian said that “Literally people coming, not even physically touching each other, but literally fleetingly coming into the same air space.” Professor Nancy Baxter (University of Melbourne) echoed that scary warning: “The spread is more likely if you’re close to the person [but] there’s still a potential for virus particles to be in the air, and breathed in by someone passing by.” Scary indeed.

However, I am not comfortable with that kind of statements which sound like a scaremongering tactic rather than enlightening the public. A relative difference between 0.01 and 0.02 (ie, 2-fold difference) is the same as between 0.40 and 0.80, but the latter is much more serious than the former. What we need to know is absolute risk, not relative risk.

What is the degree of transmission of the Delta variant? Luckily, I came across an article in the ABC, in which Professor Stuart Turville (Kirby Institute of UNSW Sydney) said that “Compared to the original strain, the Delta strain is two times more contagious” [2]. OK, it is good to know that.

However, what is the transmission rate? Well, I have over the past few days dived into the data published by Public Health England [3], and found some interesting facts. In the table below, you will find that the attack rate of the Delta variant is about 11%. In other words, if 100 individuals come into contact with an individual infected by the Delta variant, 11 are to be infected as well. This attack rate for the Alpha varian was 8%. So, I guess we can say that on average the Delta variant is 38% (eg 11 / 8) more transmissible than the Alpha variant.

What about mortality risk? Well, again the Public Health England data are really helpful. The table below shows that the rate of mortality associated with the Delta variant was 0.3% (95% confidence interval, 0.2 to 0.5%). This mortality risk is lower than that of the Alpha variant: 2%. Data from Public Health England also show that people infected with the Delta variant had lower rate of emergency admission than those with the Alpha variant (0.8% vs 1.9%).

In summary, the attack rate of the Delta variant is about 11%, or 36% higher than the Alpha variant. However, it appears that the Delta variant is less deadly than the Alpha variant. The two data points are actually consistent with the Darwinian law which predicts that most virus will eventually more contagious but less pathogentic over time. We have seen viruses such as H1N1 influenza, myxoma and OC43 behaved within that law. We hope that SARS-Cov-2 will behave in the same way, so that we can live in peace with it … forever.





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