Importation pressure and surveillance traceability: complementary indicators for sustaining measles elimination in the Americas

To the Editor,

On 10 November 2025 the Region of the Americas lost its verification as free of endemic measles transmission, following the determination by the Regional Monitoring and Re-Verification Commission that endemic transmission had become reestablished in Canada. All other countries in the region retained their national elimination status [1]. The Region therefore now contains a set of individually verified countries inside a regional space that is no longer verified.

That configuration exposes a limitation in how elimination is measured. Verification is a retrospective judgement: it requires evidence of high vaccination coverage and of interruption of endemic transmission for at least twelve consecutive months, confirmed through epidemiological, laboratory and molecular data reviewed by the Commission [2]. It is an accurate statement about what has happened. It is not a statement about how much exposure a country is currently absorbing, and for most of the past two decades the difference was immaterial, because verified countries were surrounded by verified countries.

The situation in Sao Paulo, Brazil, illustrates the gap without indicting the response. Between January and early August 2026 the state confirmed 23 measles cases, against two in the whole of 2025 [3]. Cases clustered in the state capital, Guarulhos and Sao Bernardo do Campo, along the corridor served by the largest international air hub in South America. The first two cases, in March and April, were imported and occurred in unvaccinated individuals; the remainder were classified as import-related, with the source of infection unidentified at the time of writing [3,4]. Brazil was reverified as measles-free in November 2024, five years after losing that status, having sustained interruption of endemic transmission since June 2022 [2]. The response has been prompt, with a supplementary infant dose recommended in the affected municipalities in June 2026, before the transmission chain was formally recognised [4]. A capable programme, holding verified status, nonetheless faced a chain of transmission whose origin it could not trace.

We propose that two quantities, neither captured by verification, would add prospective information at low cost.

The first is importation pressure. Poterek and colleagues showed that an inflow variable defined as the sum, across origin countries, of air passenger volume multiplied by measles incidence in the origin country predicted imported measles cases in United States states with a correlation of 0.84 and an area under the receiver operating characteristic curve of 0.78, and that air travel data and international surveillance data each contributed to that performance [5]. What we propose is a modest extension and a change of use: weighting that inflow by the susceptible fraction of the destination population, and reporting the resulting quantity alongside elimination status rather than only within predictive studies. For operational specification we would suggest a rolling three-month window; arriving passengers rather than scheduled flights, since seat capacity and load differ substantially between routes; incidence per million rather than absolute case counts, so that origin countries of different sizes are comparable; susceptibility estimated from two-dose rather than first-dose coverage; and stratification by age band where subnational coverage permits, since susceptibility is not uniformly distributed across age groups. A verified country could then state not only that endemic transmission has been absent, but how much introduction pressure it is presently absorbing.

The second is surveillance traceability, operationalised as the proportion of confirmed cases for which no epidemiological link to a known source can be established. This figure is already generated by routine case investigation in every elimination setting and is typically reported as a descriptive detail rather than as a monitored indicator. A case without an identifiable source may indicate unrecognised transmission upstream, although it may equally reflect incomplete interview data, unrecognised exposure to a known case, population mobility, or insufficiently discriminatory sequencing. It is therefore an indicator of investigation and linkage capacity rather than a measurement of surveillance sensitivity, and we propose it on that narrower basis. Its practical merit is availability: it can be computed continuously from data already collected, whereas the conventional criterion, absence of endemic transmission over twelve months, resolves only in retrospect.

Considered jointly, the two quantities describe four situations. Low pressure with high traceability corresponds to a stable elimination setting. High pressure with high traceability corresponds to a heavily exposed system that is nonetheless tracking introductions, the condition under which imported cases are contained without secondary spread. Low pressure with low traceability corresponds to a system whose investigative weakness is masked by the absence of challenge. High pressure with low traceability corresponds to a state of increased vulnerability to sustained transmission. The Sao Paulo metropolitan region in mid-2026 illustrates the type of setting this last category is intended to capture, and we note that its features were in principle recognisable prospectively rather than only from the case count.

The value of the framing is that it renders elimination continuous and prospective rather than binary and retrospective. Supplementary campaigns, supplementary infant doses and reinforcement of laboratory capacity are currently triggered by confirmed cases; indicators of this kind would allow them to be triggered by exposure, potentially earlier, before sustained transmission becomes established.

We advance this as a proposal rather than a validated instrument, and the limitations are substantial. Neither quantity has been calibrated against outcomes. The unlinked-case proportion is sensitive to investigation capacity and unstable at small case numbers, which is precisely the situation in most elimination settings. Importation pressure estimated from aggregate connectivity ignores heterogeneity in traveller susceptibility and omits land and maritime movement, a limitation acknowledged within the air travel literature itself [5] and material in several countries of the region, including Brazil. Case figures cited here derive from official state communications rather than from a published epidemiological bulletin, and the outbreak was ongoing at our data cut-off. Our claim is narrower than a method: that the status a country holds and the exposure it faces have become distinguishable quantities, that the distinction is measurable from data already collected, and that measuring it is a more useful response to regional resurgence than enumerating cases once they appear.

Declarations

Funding: none. Competing interests: the authors declare no competing interests. Ethical approval: not required; the analysis uses aggregated data already in the public domain. Data cut-off: 9 August 2026.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.