Is COVID evolving so that people can live with it, and what is the 'broken-link' model?

The coronavirus that has spread across Japan since 2020 has gone through a cycle of waves, with outbreaks occurring and then subsiding. But the specific factors behind the rise and fall in infections remain a mystery. According to research by a team that analyzed the first through fifth waves of the virus in Japan, there is a possibility that the flow of people alone may not have much of an impact on the virus's rise and fall.

During the fifth wave of infections between summer and autumn last year, the delta variant of the virus was prevalent. Daily infections in Japan neared 26,000, but from late August, around the time the Tokyo Paralympic Games opened, daily infections started to decline, and from November to mid-December, they remained below 300. No experts were able to clearly state why infections dropped off so rapidly, and many people wondered why they had.

In March 2022, the Center for Infectious Disease Education and Research (CiDER) at Osaka University released a paper describing the development of methods to analyze infection waves through mathematical models using formulas. The paper has not yet been peer-reviewed by other researchers, but below is an outline.

According to research team member Yoichi Ikeda, who specializes in theoretical nuclear physics, previous mathematical models assumed that people infected with the virus would infect others equally at a given probability, causing an exponential rise in cases. But this didn't take into account the fact that people infect others at different rates, and with such models, it was difficult to provide a good explanation as to why infections would increase and then suddenly subside.

The research team accordingly put together a "broken-link model." Under this model a person does not necessarily infect those they come into contact with at a given rate, as it is assumed that the path of infection, or "link," is broken through such actions as quarantine, wearing masks and being vaccinated, and such precautions produce differences in the rates in which people infect others. When this model was used, the team was able to successfully reproduce the transition in the number of infections from the first through fifth waves of the virus.

Considering the transition of infections in each wave, the average broken-link probability during the first wave in the spring of 2020, when a state of emergency was in effect, stood at 8%. A broken-link probability of 8% means that each infected person can be expected to infect others at a rate of 92%. After the state of emergency was declared, the flow of people was reduced by about half, but the broken-link rate did not change that much.

Ikeda pointed out, "The fact that infections subsided even though the broken link proportion stood at 8% -- meaning that infected individuals had a relatively high probability of infecting others -- suggests that limiting the flow of people doesn't have a great bearing on bringing infections under control. When people are infected or when others around them are, they become more cautious in their behavior to avoid infecting others, which leads to more broken links. It is accordingly believed that the spread of the virus can be contained even if the overall flow of people is not reduced through emergency declarations and the like."

The fifth wave also had an average broken-link probability of just 8%, but when restricted to the period during which the spread of infections subsided last autumn, the probability of broken links had risen to 11%. In terms of movement of people, data from the Ministry of Health, Labor and Welfare's expert Advisory Board, which provides advice on the coronavirus, showed that the flow of people in major cities did not significantly decline during this period. Therefore, the research team believes that the second round of COVID-19 vaccinations led to the rise in broken links.

Meanwhile, the sixth wave of infections this past winter did not quickly come under control. This can be properly explained when considering that the average probability of broken links decreased to 6% -- partly due to the infectiousness of the omicron variant, and also because the effectiveness of vaccines was wearing off.

"Rather than pouring effort into controlling the flow of people, it is better to enhance such measures as appropriately isolating infected people and temporarily switching to telework or taking other such measures if a person is infected at a workplace," Ikeda said.

Masayuki Miyasaka, a professor emeritus at Osaka University who specializes in immunology, positively appraised the broken link model, and expressed the view that the reason infections subsided during the fifth wave was that the number of people who had received their second COVID-19 vaccination shot surged.

He added, "When we look at the figures overseas, we see infection surges even when people have been vaccinated, due to people not wearing masks or not being as thorough about ventilation. Vaccinations alone cannot break the links; people also need to wear masks, avoid the three Cs (crowed spaces, confined places and close-contact settings), and incorporate proper ventilation."

Meanwhile, it appears a subtle survival strategy of the virus may play a part in the repeated waves of infections.

Viruses, which cannot multiply on their own without a host cell, change their infectiousness and virulence through repeated mutations. During the coronavirus's fifth wave in Japan, the rapid decline in the number of infected people led some to suggest that the virus had self-destructed. They were referring to a process known as "error catastrophe," in which excessive mutation results in the virus losing the ability to reproduce itself, and naturally ceasing to exist.

Most experts, however, are skeptical of this theory of self-destruction. Among them is project professor Yoshihiro Kawaoka, a specialist in virology in the Institute of Medical Science at the University of Tokyo.

"In order for many viruses to cease to exist through error catastrophe at the height of the spread of the infections, mutations causing them to self-destruct would need to occur in those many viruses at the same time, but that kind of thing is inconceivable."

At this stage, far from self-destructing, variants that are more infectious have surfaced, and omicron variants and other mutations have produced new waves of infections.

If the pathogenicity of a virus is too strong and it ends up killing the host, then it cannot increase. It is possible that in the future, the coronavirus could evolve with repeated mutations enabling it to coexist with humans without causing them too much damage.

Hiroaki Takeuchi, an associate professor at Tokyo Medical and Dental University, says that the omicron variant appears to represent an intermediate stage in the virus's evolution, as it has a higher ability to slip past people's immune defenses and is more infectious than the delta variant.

"If a variant that easily infects people and can easily multiply in the body appears in the future, it could create a new wave of infections. It's hard to imagine that the coronavirus will end with omicron, so we can't be optimistic," Takeuchi said.

Just this month, a person infected with the "XE" strain of the coronavirus, which is a combination of the BA.1 and BA.2 omicron subvariants, was confirmed in Japan. Close attention is needed to see whether the XE strain will cause a spread of infections in the future.

(Japanese original by Ayumu Iwasaki, City News Department, and Ryo Watanabe, Science and Environment News Department)