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Research in Japan helps reveal what long-lasting COVID protection vaccines offer

A man receives a coronavirus vaccine shot at a Self-Defense Forces vaccination center in Tokyo's Chiyoda Ward on June 17, 2021. (Mainichi/Yohei Koide)

TOKYO -- Breakthrough infections in which twice-vaccinated people test positive for the coronavirus have been identified across the world, and moves to provide additional "booster" shots have been spreading mainly among developed countries. These are a result of findings that vaccination-acquired antibodies that contribute to immune function reduce in number over time after vaccination. Do vaccines have lasting efficacy?

    Ordinarily when an individual is vaccinated, antibodies are created in the body. They attack invading viruses, thus preventing infection. But numerous research papers from in and out of Japan have concluded that antibody quantities fall with time after COVID-19 inoculation.

    In late August, Fujita Health University in Toyoake, Aichi Prefecture, examined 209 employees who had received Pfizer vaccines. It found that antibody amounts in their blood about three months after first vaccination had fallen to about a quarter of the amount seen immediately after their second shot. But the university's research team said that "antibodies are merely one index, and just because they decrease to a quarter doesn't mean the vaccine's efficacy accordingly decreases to a quarter."

    In fact, despite breakthrough infections, risks of developing serious COVID-19 or death have stayed low. A report by the U.S. Centers for Disease Control and Prevention (CDC) said that of the 43,127 COVID-19 cases confirmed in Los Angeles County between May and July, fully vaccinated people, who are at least two weeks out from receiving their recommended vaccine doses, have an infection rate just a fifth of that for unvaccinated individuals, and hospitalization rates for fully-vaccinated people were as low as 1/29 of unvaccinated people's rates.

    Why is it that even with decreasing antibodies and breakthrough infections, vaccinated people's severe COVID-19 case numbers have been curbed? The reason lies in the workings of the body's immune system.

    Acquired immunity, also called adaptive immunity, focusses on attacking specific viruses. After vaccination many antibodies are produced, and the body becomes able to prevent infection. The immune cells producing antibodies become "memory cells" that remember the virus's characteristics as they monitor around the body and are programmed to create antibodies if a virus invades the body again.

    Additionally, the adaptive immune system can also attack infected cells and curb viral replication. This function is also acquired through vaccination, and "memory cells" that attack infected cells are created in this process.

    The delta coronavirus strain is seen in this electron microscope photo provided by Japan's National Institute of Infectious Diseases.

    According to Ken Ishii, vaccinology professor at The University of Tokyo's Institute of Medical Science, even if a person is infected after antibodies decrease, memory cells reproduce antibodies and initiate attacks on infected cells, despite there being a time lag. He said it is likely this can curb development of COVID-19 and severe symptoms.

    Ishii said, "Vaccine efficacy has three points: preventing infection, preventing development of the disease, and preventing development of severe symptoms. Antibody decreases were predicted from the start, and evaluating the effectiveness of vaccines by looking at only antibody amounts and their performance in preventing infections is misleading."

    Work by a group at the U.S. National Institute of Allergy and Infectious Diseases (NIAID) has also hinted at memory cells' efficacy. Blood samples from 24 adults vaccinated twice with Moderna vaccines were used to examine immune responses to the coronavirus and artificial viruses given the properties of coronavirus variants. The study found that although antibody amounts in the blood decrease over time, all subjects possessed a certain level of effective immunity six months after their second shot. The group explained it was possibly because the individuals had memory cells able to respond quickly when coming into contact with viruses again or receiving additional vaccinations.

    A study by the Penn Institute for Immunology at the University of Pennsylvania and others examining 61 vaccinated individuals found that although antibodies decreased six months later, memory cells that create antibodies continued to increase, while memory cells attacking infected cells were also maintained.

    But it is possible that individuals whose antibodies did not increase after their second shot have few memory cells, creating room for concern. Masayuki Miyasaka, professor emeritus at Osaka University and an immunology specialist, said national data shows antibodies have not increased for a considerable number of the over 65s, while an even greater number of over 85s have seen no increase. It is also possible antibodies won't increase for individuals with underlying diseases including chronic kidney disease, diabetes and cancer. Miyasaka said administering a third shot to such individuals must be considered.

    "Though it's not yet known to what extent memory cells will be maintained, based on how vaccines work, it is reasonable to think they won't disappear in a short period," said Miyasaka. But he added, "Even if you have memory cells, you're still vulnerable to infections triggered by highly transmissible variants, so you must continue to take prevention measures like wearing a mask."

    (Japanese original by Tomoko Mimata and Ryo Watanabe, Science & Environment News Department)

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