After the Olympic Games, the number of nCoV infections in Japan remained dizzyingly high, pushing the health system to the brink.

In October, experts `could not understand` the reason why Covid-19 in Japan weakened so quickly.

However, according to Professor Mike Toole, an epidemiologist at the Burnet Institute, vaccination is not the only reason why the number of infections in this country has suddenly decreased.

nCoV self-destructs during the mutation process

Many experts believe that the main reason lies in the genetic changes of nCoV during the replication process, the speed is about two mutations per month.

Ituro Inoue, a professor at the National Institute of Genetics, offers a potentially revolutionary theory: The Delta variant in Japan has accumulated too many mutations with an unstructured protein, capable of correcting genetic errors.

Studies show that many Asians have a defense enzyme called APOBEC3A, which attacks RNA viruses (including nCoV) more effectively than Europeans and Africans.

Therefore, experts at the National Institute of Genetics and Niigata University discovered how the human APOBEC3A protein affects the nsp14 protein of nCoV.

Professor Inoue and his colleagues analyzed data on the genetic diversity of the Delta and Alpha variants in Japanese patient samples from June to October. They then established a model of the relationship between the sequences.

The haplotype network of the Alpha variant, which was a key factor in Japan’s fourth wave of infections from March to June, has five main groups with many branching mutations.

Initially, researchers thought that the Delta variant had a much greater degree of genetic diversity.

The Delta haplotype network has only two main groups.

This means the Delta variant in Japan is more contagious.

`We were really shocked when we discovered this,` Professor Inoue said.

Japanese people walk in the Shibuya shopping district, Tokyo, on August 7.

The premise of ending Covid-19

Mr. Inoue’s hypothesis partly explains the mysterious weakening of the epidemic in Japan.

`If the virus is still doing well, cases will definitely increase because vaccination cannot prevent breakthrough infection in some cases,` Professor Inoue said.

According to Takeshi Urano, professor at Shimane University’s Faculty of Medicine, the unexpected weakening of Covid-19 is a hot topic of discussion among many experts.

nsp14 works with other viral proteins and has an important function to protect RNA from being broken down, he explained.

The difference in Japan is that Delta has almost overwhelmed Alpha and the remaining variants since August. In countries such as India and Indonesia, variants are still circulating simultaneously.

Professor Inoue’s hypothesis can also explain why the SARS epidemic suddenly ended in 2003 without the need for any vaccines or drugs.

`This is just a hypothesis, we don’t have genomic data. But the virus disappeared and never came back again,` he said.

The question is, will Covid-19 end naturally like SARS, when the virus self-destructs and cannot multiply anymore?

`It’s not impossible, but expecting it now is a bit early. We don’t have scientific evidence yet, even though we’ve looked at data in different countries,` Mr. Inoue said.

After peaking in mid-August, daily Covid-19 cases in Japan fell below 5,000 in mid-September and below 200 at the end of October. According to Mr. Inoue, the country is in the group with a low infection rate

Some people wonder whether Japanese people possess a special gene that helps destroy Delta and nCoV in general.

`East Asians like Koreans also have similar characteristics to Japan. But their situation is completely different. I also don’t know why the phenomenon of the virus disappearing on its own is only in Japan,` Mr. Inoue said.