Tokyo may have developed herd immunity against Covid-19: Study

A new study by the Japanese researchers suggested that seroprevalence for SARS-CoV-2 rose to almost 50 per cent in Tokyo. This comes as the national capital has registered a fall in the number of cases of coronavirus.

The team wrote in their study published in the journal MedRxiv: “With the rise in SPR [seropositivity rate] nearing 50 per cent within our cohort, matching the time when Covid-19 cases waned, the possibility of herd immunity should be considered, particularly in the highly-dense urban scenario like Tokyo.”

For the study, researchers estimated seropositivity for SARS-CoV-2 throughout the summer. The team enrolled 1,877 healthy, asymptomatic employees of a large company from 11 disparate locations across Tokyo.

Japan’s fatality rate is comparatively lower than other countries, including the United States and European countries, despite loose lockdown measures. However, researchers are yet to figure out the reason behind this.

Researchers speculated that this may be related to a low SARS-CoV-2 prevalence across the general population or decreased rates of fatality among infected cases.

“Serological tests monitored across the course of the second wave can provide insights into the population-level prevalence and dynamic patterns of Covid-19 infection,” wrote the team.

Researchers mentioned in their study that the seropositivity rate increased from 5.8 per cent at the beginning of the study to an unexpectedly high 46.8 per cent by summer end.

“A high seropositivity rate in Tokyo may not be fully unexpected given its remarkably high population density, tight spacing, the widespread use of public transportation, and no implementation of a lockdown,” the team wrote further.

The team also informed that the seroreversion was not infrequent, seen in 12 per cent of participants over the one-month span between tests. “This suggests that serological testing may significantly underestimate past Covid-19 infections, particularly when applied to an asymptomatic population,” the study added.

The authors of the study believe that the coronavirus may have spread widely across the general population of Tokyo despite the very low fatality rate.

Although Japan took the atypical step of not implementing a mandatory lockdown, the second wave peaked and subsided nevertheless, they added.

“Given the temporal correlation between the rise in seropositivity and the decrease in reported Covid-19 cases that occurred without a shut-down, herd immunity may be implicated,” suggested the researchers.