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Pfizer and Moderna announced that their COVID-19 vaccines lose effectiveness over time. Vials of various COVID-19 vaccines are shown here. (Yonhap News)

The US companies Pfizer and Moderna, which produce COVID-19 vaccines using messenger RNA (mRNA) technology, announced research findings Wednesday showing that their vaccines’ effectiveness gradually wanes over time, Reuters and other news sources reported.

In data submitted ahead of discussions for US Food and Drug Administration (FDA) approval of a vaccine booster shot that day, Pfizer shared clinic trial findings indicating that the effectiveness of its vaccine began to gradually decline six to eight months after the administration of the second dose.

According to the company’s findings, the effectiveness dropped at a rate of 6% every two months. While the vaccine’s effectiveness was calculated at 96.2% the week after administration of the second dose, it fell to 83.7% four months later, Pfizer said.

Moderna shared its own findings at an investor briefing the same day.

In a study of 14,000 people given the vaccine since July 2020, effectiveness was found to decline over time, the company said.

Of the people who received the vaccine last year, 162 were found to have become infected with COVID-19, while 88 confirmed cases were counted among those who had received the vaccine since then, Reuters reported.

Only 19 of those studied developed severe symptoms, it added, explaining that the matter of whether patients experience severe symptoms is particularly important in judging a vaccine’s protection capabilities.

But the FDA indicated that the research findings were “not conclusive on whether the vaccine’s ability to stop symptomatic infection declined,” the Financial Times reported.

In explanatory materials released the same day, the FDA said that in general, the COVID-19 vaccines currently approved in the US showed adequate protection capabilities against severe cases and death.

An FDA advisory committee is set to conduct a review Friday on the vaccine makers’ request to approve booster shots.

Experts are also divided on the question of whether booster shots need to be administered to people who have received their recommended doses.

In a contribution to the British journal The Lancet on Monday, eminent researchers including FDA vaccine advisors Philip Krause and Marion Gruber and World Health Organization Chief Scientist Soumya Swaminathan said booster shots were not currently necessary for the general population.

Among the researchers credited on the contribution were major vaccine researchers in the US, the UK, France, India and South Africa.

By Shin Gi-sub, senior staff writer

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