A new mutant virus "Omicron strain" that is being confirmed in many countries and regions including Japan.

Until now, no mutant virus was more infectious than the Delta strain, but it is said that it has been replaced by the Omicron strain in South Africa.

Is it highly infectious, or will it become more severe if infected?

And is there a possibility that the "sixth wave" will occur in the Omicron strain?

I have summarized the information that I know now.

(As of December 8, 2021)

Omicron strain More infectious than Delta strain?

The new coronavirus has been repeatedly mutated, and there have been mutated viruses that are highly infectious, highly pathogenic, and easily become seriously ill when infected.

Since this summer, Delta strains have accounted for almost all of the world's coronavirus infections.

Research institutes around the world have reported 99.8% of Delta strains to the site "GISAID" where the virus gene sequence is registered even in the last two months.

No mutant virus has ever surpassed the Delta strain in infectivity.

However, in South Africa, where the Omicron strain was first reported, there are a series of reports of Omicron strain infections surpassing the Delta strain.

According to the results of genetic analysis conducted in November, it is estimated that three-quarters have been replaced by Omicron strains.

There is growing caution that it may be more infectious than the Delta strain.

Examining the genetic information, it was found that the number of mutations in the protrusion "spiked protein" on the surface of the new coronavirus was about 30, which is higher than that of the conventional mutant virus.

It is a stepping stone for the virus to infect cells, and it is thought that there are mutations that are more easily linked to cells, making them easier to infect.

There are also mutations that escape the attack of antibodies.

For these reasons, the WHO = World Health Organization has positioned the Omicron strain as the most vigilant “variant of concern = VOC”.

Is it more widespread than currently reported?

In Europe, the virus has been detected in people who entered from countries where Omicron strains have not been reported, and in some countries, genetic analysis has not been sufficiently performed.

For this reason, the National Institute of Infectious Diseases has pointed out that infection with the Omicron strain may have already spread, especially in Africa.

Experts are concerned that the infection may be spreading around the world more than we can see now, just as it was when the Alpha and Delta strains were widespread.

Some point out that "there is a risk of spreading in the 6th wave"

In Japan, the number of people infected with the new coronavirus is the lowest since last summer (2020).

However, at expert meetings of the Ministry of Health, Labor and Welfare, the spread of infection has increased due to the fact that the effect weakens over time after vaccination and that the temperature drops and activities in a closed indoor environment where infection easily spreads increase. It points out that the "sixth wave" may occur.

Experts are wary that if the Omicron strain is highly infectious, it will spread as the "sixth wave" in the season when it is easy to spread, resulting in a large spread.

When many people are infected, the infection spreads to those who are prone to becoming severely ill.

For this reason, it is said that medical care as experienced in the "Fifth Wave" of this summer may be in a critical state, and we are calling for continued infection control measures from this.

Currently, the number of people who have completed vaccination is as high as nearly 80%, which is different from the situation when the infection spread so far, but cases of infection even in vaccinated people have been reported with Omicron strains. ..

The maximum caution is taken until the infectivity and the susceptibility to serious illness (pathogenicity) are determined.

Comparison with previous mutant viruses

Based on information from WHO, the National Institute of Infectious Diseases, and public institutions in each country, what is known now, such as infectivity and pathogenicity, is compared with other "variant of concern = VOC". I summarized it.


It has been pointed out that the Omicron strain may be more infectious in terms of the number of mutations in the spike protein.

As of December 7, 2021, WHO says, "It seems that it is easier to propagate than the mutant virus so far, but it is not yet known whether this will make the infection easier to spread."

According to WHO, South Africa, which first reported the Omicron strain, has seen an increase in the number of infected people since the second week of November, more than doubling in the week at the end of November compared to the previous week. ..


According to WHO, as of December 6, all 212 confirmed cases in 18 European countries were mild or asymptomatic.

On the other hand, in South Africa, the number of hospitalized patients with the new corona increased by 82% in the week from the end of November, but the proportion of infected with Omicron strain is not clear.

On top of that, as the number of infected people increases, the number of inpatients will increase, and more information is needed to get a complete picture.

Takaji Wakita, chairman of the Ministry of Health, Labor and Welfare expert meeting, said at a press conference on December 1, "There were no deaths on charter flights from Wuhan last February. The pathogenicity is not yet known and it was infected in the city. You should think carefully, such as looking at the situation where you are. "

Dr. Fauci, chief medical adviser to the U.S. government, said in an interview with the U.S. media on December 5 that "it doesn't seem to be so severe," but it's too early to conclude. Showed the idea that is necessary.

Tomoya Saito, director of the National Institute of Infectious Diseases Infectious Diseases Crisis Management Research Center, said on December 7th at NHK's "Close-up Gendai Plus" that it may not be infected with people who are prone to become severely ill. It may take a certain amount of time from infection to aggravation, and it will take at least a few more weeks to determine whether it is likely to become aggravated, he said.

Risk of reinfection

▽ "Alpha strain"

→ Maintains the function of the antibody that suppresses the virus, is the risk of reinfection the same as the conventional strain?

▽ "Beta strain"

→ The function of the antibody that suppresses the virus decreases, the function of the cells that attack the virus is maintained

▽ "gamma strain"

→ function of antibodies to suppress the virus is slightly reduced

▽ "delta strain"

→ function of antibodies to suppress the virus is reduced

▽ "Omicron strain"

→ also reported that the risk of re-infection is likely to have gone up

WHO According to the company, there are mutations that make it easier for people with immunity to re-infect due to vaccines or past infections.

In South Africa, there is a study report that the risk of reinfection was 2.39 times higher in November than before.

However, WHO says more research is needed to determine the risk of reinfection and the extent to which breakthrough infections occur.

Vaccine effect (Pfizer Moderna RNA vaccine)

▽ "Alpha strain"

→ No change in infection prevention / onset prevention / aggravation prevention

▽ "Beta strain"

→ No

change in

onset prevention / aggravation prevention

▽ "Gamma strain"

→ No

change in

infection prevention / onset prevention / aggravation prevention not

▽ "delta strain"

does not change to → both infection prevention and develop prevention and aggravation prevention

(also reported that infection prevention and preventing the onset decreases)

▽ "Omicron strain"

→ unknown (※ also view that there is severe preventive effect)

It has been reported that the Omicron strain is infected even in people who have completed vaccination.

Symptoms are said to be mild.

In an interview with the American media, Biontech's CEO, who developed the vaccine with Pfizer, said that the Omicron strain is also likely to be effective in preventing aggravation.

Effect of therapeutic drug

Due to the mutation of the Omicron strain, there is concern that the "antibody cocktail therapy" that is administered in the early stages of infection to prevent its aggravation may be affected.

On the other hand, it is thought that there is no effect on the medicines that are taken to prevent the growth of the virus.

In addition, WHO says that drugs and steroids used in critically ill patients to prevent immune overreaction will continue to be effective.

Transmission route

The main route of infection with the new coronavirus is a small flying eyelet or a small flying eyelet that floats in a closed room called "micro-flying eyeball", and contact infection by touching the nose or mouth with a hand with the virus has also been reported. ..

The Omicron strain may be more infectious, but it is believed to be a similar route of transmission.

Expert "Don't let your guard down the possibility of becoming the source of the 6th wave"

Tetsuo Nakayama, a specially appointed professor at Kitasato University who is familiar with vaccines and viruses, said, "We have to think on the premise that we are in Japan. As for pathogenicity, it is not yet clear whether it is difficult to become severe even if infected. If the infectivity is strong as it is, it is thought that a certain number of people will become severe. Don't let your guard down. "

In addition, Professor Kazuhiro Tateda of Toho University, a member of the government's subcommittee on measures against the new coronavirus, said, "It is estimated that the infectivity is quite high, but it is still clear whether it is likely to become severe. It has not been reported yet. Although there are few reports that the disease has become severe with the Omicron strain, it is necessary to pay attention to how severe it will be in elderly people and people with immunodeficiency. Regarding the effect of the vaccine, it is necessary to look carefully. Even if it weakens, it is unlikely that it will be completely ineffective. It is important to proceed with booster vaccination quietly. Also, regarding therapeutic drugs, antibody cocktail therapy may be less effective, so it is possible. We have to wait for future data. The drug under development inhibits the enzyme that amplifies the gene of the virus and the enzyme involved in the synthesis, and even in the Omicron strain, there is no mutation in that part, so the effect is Isn't it maintained? "

Take the same measures as before

Whether the Omicron strain is a mutant virus that is more infectious and more pathogenic than the Delta strain, or is it a mutant virus that is more infectious but has a lower rate of aggravation, is still determined at the beginning of December 2021. I can't.

The origin of the Omicron strain is unknown, and it is believed that it occurred outside the reach of international virus surveillance networks.

Currently, research on infectivity and pathogenicity is underway all over the world, and WHO and the National Institute of Infectious Diseases will update the information.

The measures we can take are the same as before.

An expert meeting of the Ministry of Health, Labor and Welfare is also calling for continued basic measures such as wearing masks, disinfection and avoiding stagnation, in addition to promoting vaccination.