Since this summer, almost all of the new coronaviruses detected in the world have become highly infectious "delta strains".

Even in Japan, it has been almost completely replaced by Delta stocks since the "5th wave".

The "Omicron strain" that suddenly appeared there.

Until now, no mutant virus was more infectious than the Delta strain, but it is said that it has been replaced by the Omicron strain in South Africa.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 the most infectious mutant virus to date, or is it possible that the "sixth wave" will occur in the Omicron strain?

I have summarized the information that I know now.

Omicron strain More infectious than Delta strain?

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

Among them, Delta strains have accounted for almost all of the world's new coronavirus infections since this summer.

In the last two months, 99.8% of Delta strains have been reported by research institutes around the world to the site "GISAID" where the gene sequence of the virus is registered.

At one point, it was suspected that the infection of Lambda and Mu strains reported from South America and other countries would spread, but in the end, Delta strains remain predominant in South America as well.

No mutant virus has ever been able to infect Delta strains.

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.

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

Furthermore, when 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 said that it may be 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 the infection of Omicron strains 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.

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.

Now, the number of people who have completed vaccination is as high as 77%, so the situation is different from the case of the spread of infection so far, but cases of infection even in vaccinated people have been reported with Omicron strains. ..

We take the utmost caution until we can determine the infectivity and the likelihood of becoming seriously ill if infected.

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.

However, as of November 30, 2021, WHO states that "there is no direct scientific evidence that the infectivity is increasing."


Regarding the Omicron strain, there have been a series of reports from each country that there have been no deaths and there are many mild cases, but as of November 30, WHO said, "We do not know yet, and we are scrutinizing clinical result data." is.

Takaji Wakita, chairman of the Ministry of Health, Labor and Welfare expert meeting, said, "There were no deaths on the charter flight from Wuhan in February last year. You should think carefully. "

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.

Maintains the function of cells that attack the virus

▽ "Gamma strain"

→ The function of the antibody that suppresses the virus decreases slightly

▽ "Delta strain"

→ The function of the antibody that suppresses the virus decreases

▽ "Omicron strain"

→ The risk of reinfection increases There is a report that it may be

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"

→ unchanged in both infection prevention and develop prevention and aggravation prevention

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

▽ "Omicron strain"

→ unknown

for Omicron Co., infection in people who have completed the vaccination Cases have been reported.

Symptoms are said to be mild.

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 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.

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, "In addition to the mutation of the highly infectious" alpha strain ", there is also the mutation of the" beta strain "that escapes from antibody attack, and it is basically induced by the vaccine. I think that it has a tendency to escape from the antibody. Considering that the infection is actually spreading, I think that it is a virus that is concerned about the spread of infection. It is necessary to improve the inspection system, and if it is not possible to track close contacts, it may become the source of the sixth wave. Regarding pathogenicity, whether it is difficult to become serious even if infected I'm not sure yet. If the infectivity is strong enough, it is thought that a certain number of people will become seriously ill, so 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.

Research on the infectivity and pathogenicity of Omicron strains is underway around 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.