key: cord-0894169-484omjyt
authors: Wehrhahn, Michael C.; Millan, Gretschen; Newcombe, James P.; Long, Matthew; Keighley, Caitlin; Whiting, Paul; Chambers, Ian
title: SARS-CoV-2 IgM testing for travellers: a private pathology perspective from New South Wales and the Australian Capital Territory, Australia
date: 2022-04-16
journal: Pathology
DOI: 10.1016/j.pathol.2022.03.001
sha: a4b672510d04e9b0c06917c5fdd6c9ecd778af33
doc_id: 894169
cord_uid: 484omjyt

nan

SARS-CoV-2 IgM testing for travellers: a private pathology perspective from New South Wales and the Australian Capital Territory, Australia Sir, We read with interest the letter by Hasan et al. 1 outlining the pitfalls of relying on SARS-CoV-2 IgM in a risk stratification matrix required prior to travel by some overseas countries (Table 1 ). 2, 3 The diagnosis of acute COVID-19 relies on SARS-CoV-2 nucleic acid amplification testing (NAAT). 4 However, since 8 November 2020, the Chinese Embassy has required both SARS-CoV-2 NAAT and SARS-CoV-2 IgM serology ('dual test') be performed within 48 hours of travel from Australia to China. We agree with Hasan and colleagues that IgM detection prior to travel has currently a low sensitivity of detecting cases that are polymerase chain reaction (PCR) negative yet potentially infectious. Conversely, IgM is frequently positive beyond the infectious period and the requirement for a negative 'dual test' prior to travel is unnecessary.

Our laboratories currently use two assays that have the ability to detect IgM: the Roche Elecsys Anti-SARS-CoV-2 assay that detects nucleocapsid IgM, IgA and IgG antibody (Roche Total; Roche, USA), and the Abbott Architect SARS-CoV-2 IgM assay that detects spike antibody (Abbott IgM; Abbott, USA). These two commercial in-laboratory tests (as opposed to rapid lateral flow tests) in our in-house validation studies were found to have similar high specificities (99.5% and 98.9%, respectively) to those stated by the manufacturer (99.8% and 99.56%). Sensitivities were found to be lower (67% for both Roche Total and Abbott IgM for samples that were 8e14 days post onset of symptoms in confirmed COVID cases) than those stated by the manufacturer (85% and 86%). This may relate to the greater proportion of mildly symptomatic patients that were included in our analysis when compared to that of the manufacturer and has been described previously. 5, 6 The positive and negative concordance of the IgM also correlated reasonably well to immunofluorescence assay (IFA) IgM (78%) and Euroimmun IgA (68%) assays. While the Abbott IgM assay was positive in three cases where the IFA was negative, these were deemed to be true positives based on timing of confirmed infection. Therefore, both the Abbott IgM and Roche Total assay were found to have a low risk of false positive IgM results.

We undertook a retrospective audit of all serology performed in our two laboratories between 9 July 2020 and 19 September 2021 with a particular focus on those requiring evaluation prior to travel.

A total of 5831 samples had COVID serology performed through our laboratories with 545 (9%) specifically for pretravel testing. Sufficient history was provided in 504/545 (92%) to determine the country of destination. Of these, 224/ 504 (44%) were travelling to China. The next largest groups were flying to Samoa (n=59, 12%), USA (n=40, 8%), UK (n=26, 5%) and India (n=11, 2%).

The Abbott IgM assay was performed on 444 samples, while the Roche Total was performed on 152 samples; 101 of these samples were tested only with the Roche Total assay while the remaining 51 samples had Roche Total performed in addition to the Abbott IgM assay.

There were 45/444 (10%) positive Abbott IgM samples (with 44/44 returning a negative SARS-CoV-2 NAAT result from simultaneously collected nose and throat swabs), and there were 4/152 (3%) positive Roche Total samples, with 3/4 positive Roche samples also positive on the Abbott IgM assay, consistent with recent past infection with or without recent vaccination.

Of the 224 travellers to China, 15/190 (8%) were found to have positive Abbott IgM results with one of these also positive on the Roche assay and having confirmed infection overseas 6 months prior. This patient was eventually cleared for travel when a negative IgM result by IFA was obtained. Of the remaining 14, 10 were cleared to travel following reporting of the subsequent reflex Roche Total result which returned negative results for all of these samples. The remaining four that did not undergo reflex testing were reported as positive with further discussion with the Chinese Embassy allowing clearance due to negative PCR and recent vaccination in the previous 6 weeks considered the likely cause of the positive Abbott IgM result. Retrospective reflex testing found these four samples to also be negative on the Roche assay excluding recent infection. The remaining 34 travellers who were tested by the Roche Total alone were all negative. Since review of this dataset, an additional asymptomatic traveller to China was found to be positive by both Abbott IgM and Roche Total during the Delta outbreak in Sydney. He had not been vaccinated and, while PCR was negative, he was judged to have had recent infection. Travel was deferred and he was required to undergo repeat PCR testing (result on day 2 and day 22 negative), chest imaging (result normal) and repeat serology (3 weeks later: rising IgM and IgG levels) and travel was further delayed until his 'dual test' was negative. 7 Of the 59 travellers to Samoa, 2/42 (5%) and 0/19 were positive on the Abbott IgM and Roche Total assays, respectively, with all reported as negative (following reflex testing of the two Abbott IgM positive samples returning negative Roche Total results).

To further explore the likelihood of IgM positivity following vaccination, a random sample of 63 asymptomatic predominantly laboratory staff who had been recently vaccinated volunteered to be tested by the Abbott IgM assay. Positive IgM (Table 2) , although due to different collection schedules it is difficult to compare the difference between IgM responses following the two vaccines in our tested cohort.

Because of the ever increasing vaccination rates in our community coupled with impending opening of the borders for overseas travel, it is expected that more patients will require serology testing prior to travel. Based on our data, a majority of these might be expected to return positive antispike IgM results (up to 75% 3 weeks post the first dose of Pfizer in our cohort) with IgM detectable up to 8 weeks post mRNA vaccination in one study. 8 Recently, travellers to China therefore have been required to have nucleocapsidspecific IgM tests to at least remove those who potentially only have vaccine-induced positive IgM results.

However, COVID cases have recently increased to high levels globally due to Omicron, and it is predicted that more COVID infections will develop as elimination strategies are abandoned. Serology testing prior to travel may also result in positive nucleocapsid IgM results from actual recent infection rather than vaccination and delay travel as we saw with the recent case described above. Others have also reported concerns about false positive IgM results that have delayed travel. 9,10 Of concern is the median time to IgM seroreversion in one study 11 being 7 weeks following infection, similar to the 53% of confirmed cases still IgM positive in the 7th week post-onset of illness in another study. 12 Therefore, it is expected that significantly delayed travel following actual recent infection will be likely for a large proportion of recently infected (but not infectious) travellers if this strict requirement is maintained.

In conclusion, particularly for travellers to China, it is probable that positive IgM results related to recent vaccination or infection will only become more common. The use of a nucleocapsid based assay such as the Roche Total assay may at least reduce the proportion of recently vaccinated travellers with positive IgM results. Such an assay can be utilised as a screening assay to avoid detection of vaccineinduced spike IgM. However, an increasing number of people have been infected with SARS-CoV-2 and therefore will have nucleocapsid antibodies detectable with the Roche Total assay. In our laboratory, these are then tested with a lateral flow nucleocapsid-specific IgM assay (due to unavailability of an automated IgM-specific nucleocapsid assay at time of writing) and if positive is likely to lead to significant travel delays. Thus, the pitfalls of IgM testing have major practical implications in terms of the inconvenience and expense of postponed travel and further testing, though these requirements are unlikely to change in the foreseeable future given that they have already remained in place for the last 15 months. Like others, we can only hope that the requirement for a negative 'dual test' is abandoned and NAAT results alone will continue to be the best guide to whether an individual traveller poses an infection risk. 

SARS-CoV-2-specific IgM screening has low sensitivity for identifying potentially infectious travellers

Embassy of the People's Republic of China in the Commonwealth of Australia. Notice on airline boarding requirements for certificates of negative nucleic acid and anti-body blood tests results

Redalert: latest information on the coronavirus (COVID-19) and travel for Samoa

COVID-19). CDNA National Guidelines for Public Health Units. Version 5.1. Cited

Kinetics of viral load and antibody response in relation to COVID-19 severity

An evaluation of 4 commercial assays for the detection of SARS-CoV-2 antibodies in a predominantly mildly symptomatic low prevalence Australian population

Embassy of the People's Republic of China in the United States of America. Notice on China-bound foreign passengers' application of health code

mRNA vaccine-elicited antibodies to SARS-CoV-2 and circulating variants

Experience with pretravel testing for SARS-CoV-2 at an academic medical center

Is adding IgM antibody to polymerase chain reaction testing useful for COVID-19 travel screening?

Persistence and decay of human antibody responses to the receptor binding domain of SARS-CoV-2 spike protein in COVID-19 patients

The antibody response to SARS-CoV-2 infection

We thank Megan Yu, Deland Be and Elizabeth Marland for their assistance with laboratory testing and review of the manuscript.

Conflicts of interest and sources of funding: The authors state that there are no conflicts of interest to disclose.