A cross-sectional study published Tuesday in JAMA Network Open found that while higher levels of antibodies in vaccinated individuals resulted in lower infection rates during the delta surge, they did not have the same effect during the omicron surge, Ingrid Hein writes for MedPage Today. 

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Study details and key findings

For the study, researchers collected a serum specimen from 1,086 residents and 1,053 staff at 41 West Virginia nursing homes between September 2021 and November 2021. All participants were fully vaccinated with the Pfizer-BioNTech, Moderna, or Johnson & Johnson Covid-19 vaccine. Roughly one-fourth of the participants had also received a booster dose and 28% reported a prior infection.

Participants' average age was 67, over three-fourths were women, and 96% were white.

In total, there were 219 breakthrough infections reported from January 2021 to January 2022 among 11% of residents and 9% of staff, primarily during the delta and omicron waves.

According to study author Sally Hodder of West Virginia University School of Medicine and her co-authors, average antibody levels were twice as high among participants who did not experience breakthrough infections during the delta surge, compared to those who were infected later.

Notably, anti-receptor binding domain (RBD) IgG antibody levels decreased with time following vaccination or infection. These levels were significantly lower among fully vaccinated participants that did not have a history of Covid-19, when compared with boosted participants and non-boosted participants who previously experienced a breakthrough infection.

In addition, researchers noted that 9% of residents tested negative for anti-RBD IgG antibodies while only 4% staff tested negative.

During the delta surge, antibody index levels were a median 2.3 for participants who experienced breakthrough infections later and 5.8 among participants who never had another infection.

However, during the omicron surge, the researchers did not detect any differences between the infection and non-infection groups.

According to Hodder and her colleagues, lower antibody levels among individuals infected during the delta surge may "have resulted as a time-artifact because booster dose recommendations were initiated after onset of the Delta surge."

Notably, researchers found that anti-RBD IgG levels were higher among participants with a recent exposure, either through an infection or vaccination.

Among groups with exposures 14 to 77 days before researchers collected specimens, median antibody index levels were:

• 8.0 for fully vaccinated participants with no history of infection
• 14.0 for boosted participants with no infection
• 16.6 for boosted participants with a previous infection
• 17.7 for non-boosted participants with breakthrough infections

Commentary

According to the researchers, "[f]ully vaccinated, non-boosted individuals who had previously experienced vaccine breakthrough infections had significantly higher antibody levels compared with boosted participants with or without a history of infection."

"This observation demonstrates that booster doses may not restore antibody levels equivalently to breakthrough infections," they added. "However, given that vaccination reportedly reduced the need for mechanical ventilation and death by 94%, keeping at-risk individuals vaccinated could prove beneficial to them and help reduce resource demands associated with the management of severe SARS-CoV-2 infection."

Ultimately, the study has several limitations. First, the findings do not reflect accurate antibody levels at the time of infection since most infections were recorded two or three months after the levels were checked—but the researchers noted that participants' levels would not have been higher in these cases unless they received a booster shot after their blood test. 

In addition, the researchers noted that the results may not apply to the general population since most of the participants were white women.

"Data presented in this study are not conclusive in providing a serologic correlate of protection against SARS-CoV-2 infection," the researchers said. (Hein, MedPage Today, 9/14)