Uncovering the invisible spread of COVID-19 across the islands of French Polynesia through seroprevalence research
Imagine a string of 121 islands scattered across a vast expanse of Pacific Ocean—an area as large as Europe—where turquoise lagoons meet volcanic peaks. This is French Polynesia, a territory synonymous with breathtaking natural beauty and vibrant Indigenous culture. When COVID-19 emerged in 2020, these geographically isolated islands faced a unique challenge: how would a novel coronavirus navigate this fragmented paradise? Would their remoteness provide protection, or would it create unexpected vulnerabilities?
An intriguing scientific investigation—the MATAEA study—set out to answer these questions by tracking the invisible spread of SARS-CoV-2 across French Polynesia's archipelagoes. What researchers discovered revealed not just the dynamics of this novel coronavirus in isolated populations, but also uncovered surprising protective factors and dramatic differences between islands. Their findings, published in BMC Public Health, tell a fascinating story of how the Delta variant transformed the pandemic landscape in this unique part of the world 1 .
To understand how SARS-CoV-2 spread through French Polynesia, we first need to understand what seroprevalence means. Unlike standard COVID-19 testing that detects active infections, seroprevalence studies look for evidence of past infections by measuring antibodies in blood samples. These antibodies are like molecular memory—they remain long after the virus has been cleared from the body.
Seroprevalence studies capture both symptomatic and asymptomatic infections, providing a more complete picture of viral spread than standard testing alone.
Seroprevalence studies are particularly valuable because they capture both symptomatic and asymptomatic infections. This is crucial since many COVID-19 cases show minimal or no symptoms, especially in regions with limited testing capacity. By testing for antibodies targeting the nucleocapsid (N) protein of SARS-CoV-2 (which isn't affected by vaccination), researchers could distinguish between vaccine-induced immunity and immunity resulting from natural infection 1 .
French Polynesia experienced two distinct pandemic waves before this study. The first, caused by the original Wuhan strain, occurred between August 2020 and March 2021. The second, driven by the more contagious Delta variant, struck between July and October 2021 with dramatically greater force 1 .
Original Wuhan strain caused nearly 18,000 confirmed cases over six months
Delta variant resulted in over 28,000 cases in just three months—a significant surge in infection rate
The difference between these waves was striking. While the original strain caused nearly 18,000 confirmed cases over six months, the Delta variant resulted in over 28,000 cases in just three months—a significant surge in infection rate 1 . This aligned with global observations that Delta was substantially more transmissible than previous variants, but French Polynesia's unique geography offered a special opportunity to study how the variant behaved in a partially immune population distributed across isolated islands.
The MATAEA study was designed as a cross-sectional survey conducted on a random sample of the adult population (aged 18-69 years) representative of all five French Polynesian archipelagoes. The researchers used a stratified sampling approach to ensure all geographic areas and age groups were properly represented 1 .
Between April and December 2021, participants answered questionnaires and provided blood samples. The timing was particularly valuable as it captured the period both before and after the Delta wave swept through the islands. Importantly, the study continued despite pandemic-related challenges, with brief interruptions during the strictest lockdown periods 1 .
The scientific team used a sophisticated detection method called a recombinant antigen-based microsphere immunoassay (MIA) to identify IgG antibodies targeting the SARS-CoV-2 nucleocapsid protein. This highly specific test helped distinguish between antibodies generated by natural infection versus those from vaccination, since the vaccines used in French Polynesia (Pfizer–BioNTech and Johnson & Johnson) only trigger antibodies against the spike protein, not the nucleocapsid 1 .
| Archipelago | Participants (n) | Raw Prevalence (%) | Weighted Prevalence (%) |
|---|---|---|---|
| Society (Windward Islands) | 641 | 62.1 | 61.4 |
| Society (Leeward Islands) | 103 | 27.2 | 26.8 |
| Austral | 59 | 23.7 | 24.5 |
| Marquesas | 217 | 19.4 | 19.0 |
| Tuamotu | 59 | 16.9 | 15.6 |
| Gambier | 41 | 26.8 | 25.9 |
| Overall | 1,120 | 44.9 | 56.8 |
The study revealed that by the end of 2021, approximately 56.8% of French Polynesia's population aged 18-69 years had been infected with SARS-CoV-2. This represented a massive increase from the pre-Delta period, when seroprevalence was just 21.9%. During and after the Delta wave, seroprevalence jumped to 62.1% 1 .
This finding demonstrates just how explosively the Delta variant spread through the previously partially exposed population. Despite the earlier wave of infections and the ongoing vaccination campaign (74% of adults were fully vaccinated by the end of 2021), Delta managed to infect a substantial additional portion of the population 1 .
One of the most striking findings was that only 28.4% of infections had been officially diagnosed by healthcare professionals 1 . This means that nearly three-quarters of COVID-19 cases in French Polynesia had gone undetected—what epidemiologists often call the "iceberg phenomenon," where confirmed cases represent just the visible tip of a much larger burden of infection.
This finding has important implications for pandemic management worldwide. It suggests that official case counts—which often guide public health policies—may dramatically underestimate the true extent of viral spread, particularly in regions with limited testing capacity or where many infections are asymptomatic.
| Period of Inclusion | Participants (n) | Seropositive (n) | Prevalence (%) |
|---|---|---|---|
| Before Delta wave (April-June 2021) | 479 | 105 | 21.9 |
| During/After Delta wave (July-December 2021) | 641 | 398 | 62.1 |
| Overall | 1,120 | 503 | 44.9 |
The researchers identified several factors associated with lower likelihood of SARS-CoV-2 infection—some expected, others quite surprising:
Males had lower odds of being seropositive than females 1
Those who had never been married were less likely to have been infected 1
Smokers showed reduced susceptibility to infection 1
Individuals with diagnosed respiratory allergies had significantly lower odds of infection 1
Those vaccinated against COVID-19 were less likely to show evidence of past infection 1
The reduced risk among those with respiratory allergies was particularly intriguing. The researchers hypothesized that this might be related to "differences in the expression of the angiotensin-converting enzyme 2 (ACE2) receptor, which is used by SARS-CoV-2 to enter host cells, or to a more vigilant immune system in allergic individuals" 1 .
This finding aligns with some earlier studies that suggested allergic conditions might somehow protect against SARS-CoV-2 infection, though the exact mechanisms remain unclear and warrant further investigation.
The patterns observed in French Polynesia both resembled and differed from experiences elsewhere. In The Gambia, a study conducted during the same period found attack rates of 45% during the Delta wave and 57% during the Omicron wave—remarkably similar to the patterns in French Polynesia 3 . Also parallel was the finding that 84% of PCR-positive infections were asymptomatic in The Gambia, helping explain why so many infections went undetected 3 .
In Ghana, a seroprevalence study conducted in December 2021 found an overall seropositivity rate of 86.8%—substantially higher than in French Polynesia 9 . This difference might be explained by Ghana's lower vaccination coverage (56.9% of the target population fully vaccinated compared to French Polynesia's 74% of adults) and potentially different exposure patterns 9 .
Meanwhile, in New York City—an early global epicenter of the pandemic—seroprevalence studies showed different patterns again. Antibody prevalence increased from 13% in the first wave to 93% by the fifth wave, with dramatic increases following vaccine introduction 6 .
These comparative observations highlight how both population immunity and variant characteristics interacted to shape different pandemic experiences across the globe.
The MATAEA study provides valuable insights into how SARS-CoV-2 spread through a unique island population. Several key lessons emerge:
Perhaps most importantly, this research demonstrates the value of seroprevalence studies in understanding the true extent of pandemic spread. As the world prepares for future infectious disease threats, the lessons from French Polynesia's experience with SARS-CoV-2 may help guide more effective surveillance and response strategies.
The study also underscores how much we have yet to learn about this novel coronavirus. Why would respiratory allergies or smoking reduce infection risk? How did the virus move between remote islands? What protected the nearly 40% of the population who avoided infection even through the Delta wave? These unanswered questions remind us that science is an ongoing process of discovery—even as the pandemic phase of COVID-19 recedes, there remains much to learn from how it unfolded across diverse populations like those of French Polynesia.