Scientists Back Boosters To Stop Infection As Study Finds Waning Immunity Months After Second Dose, But Unvaccinated Are Still At Higher Risk



Protection against Covid-19 infection starts to wane just months after full vaccination, according to a new study led by researchers at Imperial College London, which scientists say underscores the need for a booster program to control Covid-19 infections as U.S. regulators meet to discuss if and when to offer additional shots of Moderna and Johnson & Johnson vaccines.     

Key Facts

Breakthrough infections were more common in people who had received the second dose of their Covid-19 vaccine between three and six months ago than those who had their second dose within the last three months, the REACT study found.

Based on data from over 100,000 swab tests taken between June 24 and July 12, the researchers found evidence of strong protection against infection for those fully vaccinated compared to the unvaccinated, who had an infection rate of 1.76%. 

However, the infection rate was higher (0.55%) among people who had received the vaccine 3-6 months ago compared to those who had received the final dose within the last three months (0.35%), the researchers found, suggesting the protection offered by vaccination faded over time. 

The researchers said the findings support the use of a booster vaccine to elevate immunity. 

The study, which has not been peer reviewed, did not examine the protection vaccination offered against severe illness, hospitalization and death, a primary purpose of vaccination that other studies suggest remains strong.

Crucial Quote

Professor Paul Elliott, the director of the REACT program, said it’s hard to say what happens to immunity after the three month point. However, ”it definitely shows incentive for people to get their booster when it becomes available.”

What We Don’t Know

The study only looked at Pfizer-BioNTech and AstraZeneca vaccines and did not look beyond the six month mark. Other vaccines in use across the U.S., Moderna and Johnson & Johnson, also show signs of waning immunity over time and regulators are discussing the potential use of booster shots for these vaccines this week.  


By measuring infections, the study provides better evidence of waning immunity than many studies that endorse boosters, which typically infer immunity with suggestive, though not complete, proxies like antibody levels in the blood. However, the researchers only looked at how many people tested positive for Covid-19 and did not account for whether those who tested positive had symptoms and, if so, how severe these were. While preventing infection is important for controlling the spread of disease, a crucial purpose of vaccination is to prevent serious illness, hospitalization and death. Experts believe there is strong evidence the vaccines still achieve this and that premature booster campaigns threaten significant side effects in those receiving them. 

Key Background 

The U.S. approved booster shots for immunocompromised people who received the Pfizer or Moderna vaccines in August and announced plans to roll out boosters to the general public in September. The CDC gave the go ahead for the Pfizer shot to be used as a booster jab in seniors, high-risk workers and the clinically vulnerable, a much broader approach than that recommended by FDA experts, who voted against the extra shot for adults in high-risk jobs. The issue has sparked fierce debate among health officials and is set to be reignited this week as the FDA considers boosters for J&J and Moderna vaccines. The rollout of booster shots has outraged public health leaders around the world, who brand the decision as unfair given the vast inequalities in vaccine access around the world. Several high-ranking FDA officials were co-authors of a paper in the Lancet medical journal arguing against the need for boosters in the general public at the moment, citing a lack of evidence they are needed and the fact that the scarce supplies would be better used vaccinating people who have not been fully vaccinated than giving extra doses to those who already have. 

Further Reading

Scientific Evidence Doesn’t Back Booster Covid Shots, Researchers Warn — Even For The Delta Variant (Forbes)

Booster shots for COVID-19—the debate continues (Lancet)

COVID vaccine immunity is waning — how much does that matter? (Nature)

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