Omicron has reduced its virulence, why does Wu Zunyou say that the death toll is higher than that of Delta in the same period?

At a press conference held by the Joint Prevention and Control Mechanism of the State Council on April 6, Wu Zunyou, chief epidemiologist of the Chinese Center for Disease Control and Prevention, introduced that the conclusion that "the number of deaths caused by the Omicron strain epidemic is higher than the number of deaths caused by the Delta strain epidemic in the same period" is a re-analysis of foreign public data.

For example, in the United Kingdom, the Delta strain epidemic was predominantly from August to October 2021, with a mortality rate of 16 deaths per 100,000 population, while from November 2021 to January 2022, the Omicron strain was predominantly endemic, with a mortality rate of 22 deaths per 100,000 population.

The U.S. mortality rate is 40 deaths per 100,000 population in August-October 2021 and 42 deaths per 100,000 population between November 2021 and January 2022.

Compared with foreign countries, the mainland has a low case fatality rate of new crown pneumonia. From January to March this year, more than 100,000 cases were reported in the mainland, including 197 cases of severe and critical illness. The reason for the low case fatality rate is that the mainland has been taking a number of measures to prevent or reduce deaths.

What do you think of this passage that Wu Zunyou said? Hokkaido University Master of Neuroscience, a senior medical consultant of Sheng Nuo, Weibo Science Popularization University V Zhuang Shilihe made an interpretation.

Zhuang Shilihe: First of all, let's talk about some of the terminology of epidemiology - mortality and mortality rate.

Many media reports often mix these two terms, including some popular science articles for ease of understanding, but in fact, there is a huge difference between the two.

Mortality rate, which represents the proportion of people dying from a disease in a population over a certain period of time (usually in years), is the most commonly used indicator of the risk of death in a population.

The case fatality rate (CFR) is a measure of the risk of a patient dying from a disease as a percentage of the number of people who die from a disease over a certain period of time.

In simple terms, the denominator of mortality is the population, while the denominator of the mortality rate is the patient; mortality is usually used to measure the impact of a disease on the entire population over a specific period of time, and the mortality rate is usually used to measure the risk of a disease to the individual patient.

Let's expand on this latter point. Just because a disease has a high mortality rate does not mean that its mortality rate is also high.

For example, rabies, everyone knows that its case fatality rate is extremely high, but due to the low incidence of rabies, its mortality rate is not as good as AIDS.

In 2020, there were 188 rabies deaths in China, compared with 18,819 deaths from AIDS, with the former having only one percent of the latter.'s. That said, rabies affects the general population in China less than AIDS.

However, rabies is still an extremely lethal disease. In addition, the course of AIDS is much longer than that of rabies, and as long as HIV-infected people receive antiretroviral therapy in a timely and standardized manner, their life expectancy can be close to normal people; while the course of rabies is usually only 1 to 3 months, rarely more than 1 year.

So you should know the difference between the two by now.

When we want to look at the impact of a disease on the whole population in a certain period of time, or the overall effect of epidemic prevention work, we usually choose mortality as a reference indicator;

When we look at the severity of a disease, or whether our drug interventions are effective, the case fatality rate is of greater reference value.

So back to the topic of Delta and Omicron.

More detailed data, Wu Zunyou has been mentioned at the press conference, we will take one of the examples to illustrate.

A curve for new infections and deaths per day in the United States

Let's start with the case fatality rate.

The case fatality rate of the new crown virus depends mainly on several factors: age, vaccines and drugs, and variants.

Age: the most important factor influencing case fatality rates, which increase exponentially as we age.

Vaccines: Complete vaccination (and booster injections) can significantly reduce the risk of severe illness and death.

The above two points are fully reflected in the real-world data of various countries and regions, such as the data of Singapore and Hong Kong.

Drugs: Subject to capacity bottlenecks, the role of drugs has not yet been fully exerted, but some oral antiviral drugs have shown strong protection in phase III clinical trials, significantly reducing the risk of hospitalization or death in high-risk outpatients.

Variants: From the original strain of the new crown, Alpha to Delta, the infectivity of the virus is increasing, but the toxicity has not decreased significantly, and the significant weakening mainly occurs in Omicron, which weakens the ability of Omicron to fuse with some lung epithelial cells, which is an important factor in its decreased pathogenicity.

However, it is also important to weaken the pathogenicity of Omicron, a large number of people have been infected with or vaccinated with previous variants, and the body already has protection, which can reduce the risk of severe illness after infection.

So even if the strain is completely unchanged, over time, the overall immunity of the population due to infection will increase, and the pathogenicity of the virus will gradually decrease (of course, this is ideally, the virus can not not evolve).

So in a study published at Cornell University earlier this year, for example, the Omicron case fatality rate fell from 5.3% to 0.9% during the Delta outbreak (this is a preliminary study, there is a lag in deaths).

According to a study released by the Office for National Statistics in late February, Omicron's risk of death fell by 67%.

At present, the vast majority of studies in various countries show that the case fatality rate of Omicron is significantly lower than that of Delta due to both intrinsic and extrinsic factors.

Above is the difference in case fatality rates. But when we focus on mortality, the situation is different.

As mentioned above, the mortality rate is for the entire population of an area, and when there is a pandemic in this area and many people are infected, the death toll may go up.

Compared to any previous wave, Omicron caused the biggest wave of outbreaks since the COVID-19 pandemic in most countries, and we can see from that map in the United States.

The same situation has also occurred in some previously controlled countries or regions that were relatively well controlled. In the past two years, the epidemic control in Hong Kong, China, has been very good, but in the face of the highly contagious Omicron BA.2, Hong Kong's defense line has been quickly broken. The number of deaths in a single day in Hong Kong in March directly exceeded that of the past two years combined.

In addition to the strong contagiousness of Omicron, another characteristic of it is that the immune escape ability is very strong, which is reflected in the topic we are going to talk about today, for the same people who have been vaccinated with two doses of vaccine, they may be able to carry Delta, but it is likely to be unable to carry Omicron.

In Florida, for example, older adults accounted for about 60 percent of deaths during the Delta pandemic, while Omicron rose to about 80 percent.

The significance of only two injections for young people and only two injections for the elderly is different, as evidenced by the results of a real-world study released in Hong Kong last month, and the three-shot vaccine can significantly improve the protection of the elderly against death.

But it was too late for the people of Hong Kong to understand this truth, and Hong Kong became the region with the highest number of new deaths per million people in the world. The elderly who died in this wave of the epidemic in Hong Kong have taught the world the most tragic lessons.

This is what happens when a virus with low pathogenicity is in full swing. Even though each case has a lower risk of severe illness and death, when you multiply that risk by a very large number of infections, you get a sizable death number.

So infectious diseases and epidemiological principles, the number of outbreaks in countries during the pandemic, and various real-world studies tell us that Omicron is not a big flu, and it is far more destructive to the world than seasonal flu – although for a person who has been given a booster injection, its symptoms may be less than that of influenza.

Clarifying the concepts of case fatality and mortality is our first step in confronting Omicron. Whenever an outbreak breaks out, the most vulnerable groups in the population tend to be the most at risk, such as the elderly and those with underlying medical conditions.

So although I've written this sentence countless times, I'll repeat it here – go to the vaccine as soon as possible if you haven't been vaccinated, and get a booster injection as soon as possible.

For example, some of the consultations I often receive about whether patients with tumors and those who use immunosuppressants can be vaccinated, but in fact the risk is not safety.

There have been many studies that have shown that the safety of vaccination in these populations is not significantly different from that of ordinary people, and the risk mainly comes from protection, because there are some drugs that inhibit the protection of vaccines, but there is a little protection that is much stronger than no protection at all.

In addition, in the face of the highly contagious Omicron, there is also a very real problem is the impact on the medical system. The number of medical staff in the whole society is relatively stable, and the entire medical system is not specifically designed to deal with a specific infectious disease, and usually needs to receive a variety of acute and chronic diseases and pregnant women.

Under the Omicron outbreak, how to rationally allocate medical resources has become the most difficult problem, because if most of the medical staff are pulled to support the fight against the epidemic, it will inevitably lead to a large number of patients with other diseases without effective treatment, and the case fatality rate of some of these diseases is much higher than that of Omicron.

So the run on the health care system will eventually lead not only to an increase in COVID-19 deaths, but also to an increase in excess deaths – that is, an increase in the number of deaths from all causes compared to the previous year.

At this level, it's not just a matter of comparing Omicron and Delta case fatality rates, it's a more complex one. How to reasonably triage people with different symptoms of COVID-19 and how to allocate medical resources to try to maintain the needs of other patients are difficult problems for many countries.

Finally, for yourself, personal protection is the greatest contribution to society, wash your hands and wear a mask frequently, and if you have been vaccinated with a booster, the threat of Omicron to your health is actually very small. It is the immune barrier composed of countless ordinary people that blocks the spread of the disease, which is the real herd immunity.

Planner: Cat, z_popeye

Executive Producer: Gyouza