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U.S. firm Pfizer Inc. has called for an additional third shot of its COVID-19 vaccine, citing new data showing its efficacy trails off after six months, but doctors remain unconvinced, saying that what’s more important now is to focus on inoculating those who haven’t received all of their shots yet.

Manufacturers of messenger RNA (mRNA) vaccines administered in Japan — Pfizer and its German partner BioNTech SE, as well as U.S. firm Moderna Inc. — have been developing booster shots in the belief that a third dose will be necessary sometime between six and 12 months after people get their second shot. The issue of additional shots comes amid an alarming spike in new infections globally, driven by the highly transmissible delta variant.

Pfizer’s clinical study — based on around 42,000 volunteers in six countries and published on the medRxiv preprint server for health sciences Wednesday — showed that the efficacy of the vaccine was 96.2% from seven days after a second dose. It then fell 6 points by two months after the second dose and another 6 points four months after, and finally to 83.7% in the four- to six-month period.

Pfizer said it plans to ask the U.S. Food and Drug Administration (FDA) in August for emergency authorization of a booster shot, saying it elicits antibody levels against the delta variant that are more than fivefold higher in younger people and more than elevenfold in older people than after a second-dose regimen.

But the announcement has been met with a cold shoulder from experts at home and abroad.

“Vaccine-acquired immunity declines, but it still shows high efficacy,” said Dr. Tetsuo Nakayama, a project professor at Kitasato Institute for Life Sciences and director of the Japanese Society of Clinical Virology. “COVID-19 patients infected with the delta variant are mostly young generations who have not had a vaccine. At present, I see no need for a booster shot, and I’d rather think that we should emphasize the completion of two doses for these generations.”

A man is vaccinated in Izunokuni, Shizuoka Prefecture, in July 19. | POOL / VIA KYODO
A man is vaccinated in Izunokuni, Shizuoka Prefecture, in July 19. | POOL / VIA KYODO

Citing the hugely uneven global gap in vaccine supply, World Health Organization chief Tedros Adhanom Ghebreyesus has also pushed back against booster shots, saying that about 75% of vaccines have been administered in just 10 countries, while only 1% of people in low-income nations have received at least one dose.

“In general, even if antibody levels have declined, exposure to the pathogen immediately calls back the immunologic memory, leading to the production of antibodies,” said Dr. Hiroyuki Moriuchi, professor of pediatrics at Nagasaki University. “Therefore, unless the antibody levels were much lower, I don’t think there’s a need for a booster shot.

"The ones we actively need to consider for a third vaccination are people with suppressed immune systems — which are linked to lower efficacy of the vaccine — who are at risk of serious cases of COVID-19.”

Some countries, such as Japan, have already made preparations to secure a booster shot if and when it becomes available. The health ministry last week signed a contract to get 50 million additional doses of the Moderna vaccine — to be supplied from as early as the beginning of next year — allowing Japan to get deliveries of the firm's vaccines and its updated booster candidate.

Israel this month began offering a booster to some people with weak immune systems, and the country is considering whether to expand the program to other people, especially those who are older.

Despite almost half of Americans being fully vaccinated, the pace of the rollout there has stalled recently, hampered by those who are reluctant to get a shot or refuse to do so.

A group inoculation for junior high school students in Soma, Fukushima Prefecture, on Tuesday | KYODO
A group inoculation for junior high school students in Soma, Fukushima Prefecture, on Tuesday | KYODO

The consequences are huge. The U.S. has seen a surge in its weekly average for daily new infections to nearly 64,000 cases, the highest since April, according to Our World in Data. A similar resurgence in new cases is also developing in other countries such as the U.K., which has coincided with the rise of the highly transmissible delta variant globally.

That variant has been fast spreading among the unvaccinated population worldwide, and the U.S. Centers for Disease Control and Prevention (CDC) and the FDA believe virtually all COVID-19 hospitalizations and deaths are among people who have not been inoculated. In comparison, chances of “breakthrough” infections after getting a second shot are still considered rare, and fully vaccinated people are effectively protected against severe disease and death.

Following a recent surge in new cases, Dr. Anthony Fauci, the top U.S. infectious disease expert, said Sunday that people in the U.S. could need a booster shot. Meanwhile, the CDC on Tuesday tightened its indoor mask policy, saying that even fully vaccinated individuals should resume wearing a mask indoors in public in parts of the country with substantial transmission areas.

But the European Medicines Agency and the CDC and FDA maintain that booster shots are not needed for now, with the U.S. regulators calling on unvaccinated people to get their shots as soon as possible to protect themselves and their community.

The scientific consensus is that a booster shot will be needed only when new variants emerge that significantly raise the number of severe cases or deaths among fully vaccinated people.

Scientists say two doses of Pfizer and Moderna’s vaccines stimulate a powerful immune response composed of T-cells in the blood, which bind to and kill infected cells, and memory B-cells, which produce antibodies. Together, these could possibly provide protection against severe disease and death for years.

A medical worker prepares a dose of the Pfizer-BioNTech COVID-19 vaccine at a vaccination center in Paris on Friday. | REUTERS
A medical worker prepares a dose of the Pfizer-BioNTech COVID-19 vaccine at a vaccination center in Paris on Friday. | REUTERS

One good indicator of the strong protection offered is the level of neutralizing antibodies in the blood. The antibody levels of coronavirus vaccines tend to remain higher over a longer period compared with conventional vaccines, health experts say.

Exactly how long the effect of the vaccinations lasts is unclear, Moriuchi of Nagasaki University said, although some clinical studies have shown that the effect of the vaccination lasts for at least six months.

Moriuchi said that lab tests of the antibody levels in the blood of people who have received two doses of an mRNA vaccine have shown that the neutralizing antibodies are much more powerful than those induced by conventional influenza vaccines.

“Even if the blood of those people gets diluted 500 times on average, it can hold back the conventional coronavirus or alpha variant,” he said. “For the delta variant, blood diluted up to 150 times on average still works to inhibit the virus … so the effect should last much longer."

In comparison, individuals who have received an influenza vaccine have much lower protection against that disease, as blood diluted more than around 10 times on average would not protect against the disease, Moriuchi said.

And given the demographics in which the virus is spreading, inoculating young people needs to be the focus, said Nakayama of the Kitasato Institute for Life Sciences.

"Accelerating the rollout of two doses should come first, rather than a third shot to curb the infections more effectively.”

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