BY MAGDALENA OSUMI
Japan has so far managed to avoid a coronavirus explosion as seen in other countries, but its efforts towards containing the virus raise the question: Will Japan manage to reduce the risk for infection to a level comparable to that of other infectious diseases, such as chickenpox or measles, that were once very common, too?
Such a scenario could materialize if a significant percentage of the population achieved herd protection against the disease. But is Japan close to reaching it?
This article examines the basic concept of herd protection or herd immunity and Japan’s progress towards it.
What is herd immunity?
Herd immunity refers to the epidemiological concept of building immunity in a population. When most of a population is immune to a disease, this provides herd immunity, a form of indirect protection to those who are not immune in the community.
The importance of herd immunity was first recognized with smallpox, where the initial goal was to immunize 80 percent of the population to achieve such an effect.
According to Johns Hopkins University, depending on how contagious a virus is, usually 70 percent to 90 percent of a population needs immunity. For instance, if 80 percent of a population is immune to the coronavirus, four out of every five people exposed to it won’t get sick and thus won’t further infect others, making it easier to control.
Herd immunity can be also reached through administration of vaccines, which in the case of the new coronavirus are not yet available.
How can herd protection be gauged and what’s the role of antibody testing?
Authorities in many countries including Japan have begun mass antibody testing to gauge the extent of the virus’s spread and whether they are approaching herd immunity. The results are also expected to help authorities develop an outlook for infection numbers if a resurgence takes place, draft strategies to prevent another wave and estimate how many people will need to be vaccinated.
The antibody tests, also known as serology tests, are being conducted to determine whether people contracted SARS-CoV-2, the formal name of the virus that causes the respiratory disease COVID-19.
The screenings are designed to detect antibodies, or proteins, in a blood sample that are formed when responding to an infection. An antibody test does not assess whether the virus is currently present and is performed after one has fully recovered from COVID-19.
The tests have also raised expectations that antibody tests could become an important aid in diagnosis, as the capacity limitations of the popular polymerase chain reaction tests being conducted on mucus and saliva samples mean the actual scale of the virus’s spread may be hidden.
Kazuhiro Tateda, an expert on infectious diseases who is a professor at Toho University, has warned that if the infection had spread without being detected, “the rates would have been higher.”
Tateda, who heads the Japanese Association for Infectious Diseases, recently called for antibody testing to be conducted periodically to grasp the spread of the virus, saying, “While we managed to survive the first wave, many people remain vulnerable to infection.”
However, the accuracy of the tests remains a challenge for health care professionals. The health ministry used a method to measure antibody levels that was approved in the United States as an emergency step. But there is no standardized antibody testing method because clinics and companies, both there as well as here, are introducing such tests independently. Their results may not be accurate even if people test positive.
If you have antibodies, does it mean you’re immune to COVID-19?
Positive results may mean that you have some immunity, and some experts have said that in healthy people with fully developed antibodies, reactivation is rather unlikely.
Reports on coronavirus infections, including those in Japan, have shown that the virus can be detected in patients who have recovered from the disease. Masaya Yamato, director of the Infectious Diseases Center at the Osaka-based Rinku General Medical Center, said such a phenomenon can be attributed to reactivation of the virus. Experts say that such a scenario is likely in patients who have not yet produced antibodies or those who are immunodeficient, for instance transplant patients or people infected with HIV whose adaptive immune system is missing.
But according to the World Health Organization, there’s still a lack of evidence on whether having antibodies means you are protected against reinfection from COVID-19.
Can mass testing help?
Preliminary results from studies of such serological tests in the U.S. and other regions have made headlines and raised hope the results can serve as a clue on how the coronavirus behaves.
Large antibody tests have already been carried out in the United States. Additionally, Germany, which was the first country in Europe to run such tests, is aiming to assess COVID-19’s spread by testing the entire population in the following months.
How close is Japan to achieving herd immunity?
Reaching herd immunity seems to be a very long shot.
Japan earlier in June conducted tests on 10,000 randomly selected people in Tokyo, Miyagi and Osaka prefectures age 20 or older to detect infections, including in asymptomatic people. The results suggested a 0.1 percent infection rate in Tokyo, 0.17 percent in Osaka and 0.03 percent in Miyagi.
As of Wednesday, Japan had around 18,000 confirmed infection cases and over 950 deaths. So far, the number of reported cases accounts for barely 0.01 percent of the population.
Given that at least 70 percent of the population would need immunity to achieve herd immunity, it will certainly take Japan years to get close to the threshold.
While Japan and most countries in Europe and Asia adopted a strict approach to control the spread of the virus, Sweden’s soft strategy without lockdown or strict social distancing rules — leaving schools, shops and restaurants open throughout the pandemic — was expected to bring the nation closer to herd immunity.
But a recent study conducted for Sweden’s Public Health Agency, has shown that only 6.1 percent of the population had developed coronavirus antibodies by late May. Sweden’s lax response to the pandemic backfired, as the country has seen more than 60,000 infections and some 5,000 deaths, with a mortality rate estimated at 39.57 deaths per 100,000 residents.