Immune response to COVID-19 vaccines declines after two months, study finds

Immune response to COVID-19 vaccines declines after two months, study finds
Protection from the two-dose COVID-19 vaccines available in the United States starts to wane as early as two months after the second shot, including among previously infected people, according to new research. File Photo by Debbie Hill/UPI | License Photo

Aug. 30 (UPI) -- Antibody levels drop by 20% two months after receipt of the second dose of the COVID-19 vaccine in adults previously infected with the virus, a study published Monday by Scientific Reports found.

Prior exposure to the virus also does not guarantee high levels of antibodies against it, the data showed.


Additionally, those infected with COVID-19 don't produce a "robust" antibody response after the first dose of the vaccine, an indication that both shots are needed even in those who were sickened earlier in the pandemic.

These findings suggest that people who have had mild or asymptomatic illness with COVID-19 have essentially the same antibody response to vaccination as those who have not been previously exposed, the researchers said.

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"Many people, and many doctors, are assuming that any prior exposure to [COVID-19] will confer immunity to re-infection," study co-author Thomas McDade said in a press release.

However, "our study shows that prior exposure to [the virus] does not guarantee a high level of antibodies, nor does it guarantee a robust antibody response to the first vaccine dose," said McDade, a professor of anthropology at Northwestern University in Evanston, Ill.


The findings are based on an analysis of blood antibody levels in 27 people diagnosed with COVID-19 earlier in the pandemic and who later received one of the available two-dose vaccines from Moderna and Pfizer-BioNTech.

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Levels of antibodies -- cells produced by the immune system to fight off viruses -- were assessed following both the first and second doses, according to the researchers.

Using at-home antibody testing kits developed by the researchers, participants submitted blood samples two to three weeks after their first and second vaccine doses and two months after the second dose, the researchers said.

In the lab, the researchers tested for neutralizing antibodies by measuring whether participants' blood samples could block the coronavirus' ability to cause an infection.

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"When we tested blood samples from participants collected about three weeks after their second vaccine dose, the average level of inhibition was 98%, indicating a very high level of neutralizing antibodies," McDade said.

However, two months after participants were given the second dose of the vaccine, the antibody response declined by 20% and varied based on history of prior infection.

Participants with confirmed cases of COVID-19 and multiple symptoms had a higher level of immune response than those who tested positive but had mild symptoms or no symptoms, the data showed.


In addition, the level of antibody response following vaccination was significantly lower for virus variants that originated in South Africa, England and Brazil, and ranged from 67% to 92%.

The Delta variant, which is currently the dominant strain of COVID-19 in many parts of the United States and originated in India, was not part of the study.

Research has suggested that getting infected with the coronavirus created long-term immunity against it, and previous infection could mean only needing one dose of the two-shot vaccines to bolster this protection.

The new study suggests, however, that even those who have been sickened with the virus may still be susceptible to it, and that both vaccine doses may be needed, particularly as new variants emerge, the researchers said.

"As far as protection goes after vaccination, the story is the same for all the variants, including Delta -- the vaccine provides good protection, but not as good protection as the original version of the virus for which the vaccine was designed," McDade said.

"Combine that with the fact that immunity wanes over time, you get increased vulnerability to breakthrough infection," he said.

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