People with a smaller weight may maintain their immunity against COVID-19 for longer than those who are heavier, according to new research.
A small peer-reviewed study in South Korea has found a correlation between weight and immunity in individuals who have been vaccinated against COVID-19, which is caused by the SARS-CoV-2 virus. It found that people who are lighter - weighing less than 55kg - may maintain higher levels of antibodies over time compared to those who weigh more. Antibodies are large proteins used by the immune system to identify and neutralise foreign objects, such as pathogenic bacteria and viruses.
The team of researchers from Kyungpook National University in Daegu tested the blood of 50 healthcare workers two, four and six months after their second dose of the Pfizer-BioNTech vaccine to measure their waning immunity. The healthcare workers, 10 of whom were men and 40 were women, had no prior history of COVID-19 infection.
The study, which was published in the medical journal JAMA Network Open on Thursday, found only 11 percent of participants under 55kg had fewer than 80 percent of their antibodies remaining at the six-month mark - compared to 42 percent of those who weighed more than 55kg.
"In this cohort study of 50 individuals, anti–SARS-CoV-2–specific antibody levels at two, four and six months after COVID-19 vaccination were inversely correlated with body weight. Young and middle-aged healthy adults weighing less than 55kg maintained a high antibody level six months after the second dose," the researchers said.
"Further studies are needed to clarify the antibody cutoff level to protect individuals from severe infection."
All 50 participants acquired anti-SARS-CoV-2 antibodies and maintained a positive level (more than 30 percent) for up to six months after their second injection, the researchers noted. Although the average level of antibodies waned with time, just how much it decreased was found to be correlated with weight, body mass index, body fat amount and weight-to-height ratio.
"A 1-SD [one standard deviation] increase in body weight, weight-to-height ratio, and body mass index was associated with a 4 to to 5 percent decrease in anti–SARS-CoV-2 antibodies in… women," the researchers said.
"The probabilities of less than 70 percent and less than 80 percent antibody at six months were 0 percent and 11 percent in participants weighing less than 55kg respectively, but 16 percent and 42 percent in participants weighing 55kg or greater."
The researchers concluded that a booster shot of the Pfizer-BioNTech vaccine could be administered more than six months after the second dose in young and middle-aged healthy people with a low body weight. In New Zealand, fully vaccinated people aged 18 and over become eligible for a booster three months after their second dose.
According to Te Ara: The Encyclopedia of New Zealand, as of 2009 the average man in New Zealand weighed 84.7kg and the average woman weighed 72.1kg. Statistics compiled by the Bloomberg news service in 2012 put the average Kiwi woman's weight at 74.6kg and Kiwi men at 88.54kg.
Other figures from a 2011 University of Otago and Ministry of Health nutrition survey put the mean weight of Kiwi men at 85.1kg and women at 72.6kg.