Serology data shows only 2% of cases were reported in the first year of the pandemic, and only 10% last year.
The true number of covid cases around the world “far exceed” reported cases, according to a major new analysis of seroprevalence data.
The analysis of more than 5 million participants in 100 seroprevalence studies published between early 2020 and mid-2022 showed that only 2% of covid cases were reported in the first year of the pandemic, rising to only 10% last year.
By late 2021, high-income countries in the Americas had the best ratio, but still only recorded slightly more than half of the true covid infections, according to the study in PLOS Medicine.
“Currently, our global estimates of infections based on seroprevalence far exceed reported cases captured by surveillance systems,” the authors wrote.
The analysis also found dramatic increases in seroprevalence due to either infection or vaccination over the last few years.
In June 2020, seroprevalence was estimated at 7.7%.
By September 2021, it was 59.2%. Just over half of this was attributable to infection, the study found.
In the wake of Omicron in March 2022, seroprevalence due to infection was 47.9% in high-income countries in Europe, and 33.7% in high-income countries in the Americas.
By then, total seroprevalence reached as high as 95.9% in high-income European countries and 99.8% in the Americas.
“As we enter the third year of the covid-19 pandemic, implementation of a global system or network for targeted, multi-pathogen, high-quality, and standardized collaborative serosurveillance is a crucial next step to monitor the covid-19 pandemic and contribute to preparedness for other emerging respiratory pathogens,” the authors wrote.
Stringent public health and social measures in some regions were associated with lower seroprevalence, they said.
Young adults in their 20s were significantly more likely to have antibodies than children under 10 and adults over 60, the authors said.
“Lower seroprevalence in adults 60+ could be explained by immunosenescence that can lead to quicker seroreversion higher mortality and hence a lower proportion of individuals with evidence of past infection, gaps in vaccine access, or more cautious behaviour resulting in fewer infections in this age group,” they wrote.
“There are several possible explanations for lower seroprevalence in children: milder infections, which are generally associated with lower antibody titers; school closures; and ineligibility for vaccination.”
The authors said that while antibodies persisted in most people for 12 to 18 months, the risk of reinfection with the Omicron variant was reported to be much higher than previous variants in both vaccinated and previously infected people.
That indicated that the presence of antibodies was less protective against infection, they said.
“However, seroprevalence estimates remain indicative of protection against severe disease, as cellular immunity is unlikely to be disrupted even with an immune-escaping variant of concern.”
