Reply to “Letter to the Editor: Pitfalls in Calculating the Incidence of GBS During the Pandemic”

Abstract

We read Finsterer's letter with interest. No pitfalls were encountered in calculating GBS incidences, but our study presents limitations already discussed in the original article.

First, Finsterer suggests including other codes than G61.0 for GBS diagnosis, such as polyradiculitis, radiculopathy, or polyneuropathy, because SARS-CoV-2 GBS can have different clinical manifestations than classic GBS. If we include those pathologies and considering that this is a retrospective study performed through a database designed to inform Public Health policies that does not allow clinical chart review, we would be including infiltrative polyradiculitis, mechanic radiculopathies, and diabetic polyneuropathies in the calculation of GBS incidences. We are aware that one important limitation of our study is the inaccurate codification because diagnoses are not confirmed later from discharge in each hospital, but GBS misdiagnosis rates are low and likely remain stable across all studied years. Furthermore, the literature indicates that SARS-CoV-2 GBS does not exhibit any clinical characteristics that differ from those of classic GBS [1, 2].

Additionally, pre-pandemic GBS incidences in Spain are completely aligned with previous reports on GBS incidence, which provides reassurance on the solidity of the findings and the codification choices.

On the other hand, we agree with Finsterer's statement that the decrease in incidence in 2021 may be due to preventive measures and we discussed this point in our article. Besides, this is why the incidence in 2021 was high in December but low in January. If we look at Figure 2, we can see that GBS incidences in January, February, and March of 2021 are considerably lower than in the same months in 2018–2020, which coincides with the first winter after the pandemic, when the prevention measures were stricter, and masks were widely used. We also agree that some mild cases may not be hospitalized due to restriction measures.

Also, we cannot differentiate between GBS associated with SARS-CoV-2 infection and GBS associated with vaccination because data on previous infections or vaccines at the individual level are lacking in the database we used. The effect of the SARS-CoV-2 vaccination was indeed studied, but it was withdrawn from publication following the recommendations of the journal's reviewers because the interval between vaccination and GBS onset was not available and the vaccination in Spain mostly occurred with mRNA-based vaccines (not associated with an increased GBS risk [3, 4]). Nonetheless, readers can review the results of vaccines and GBS in the preprint published on medRxiv [5]. The results illustrate that mass vaccination of the Spanish population has not increased the incidence of GBS significantly at the population level, which reinforces the population-wide safety of vaccine administrations even if a minor increase in vaccine-associated GBS is present. To improve this point, it would be very interesting to have epidemiological surveillance systems that include vaccination data at the individual level in our country.

To conclude, we do not see any pitfall or mistake in calculating GBS incidences; our article has some limitations due to the nature of the data extracted from a coded database with no clinical purpose. Implementation of real-time nationwide registries with individualized information would help to understand post-infectious disorders such as GBS.

L.Q. received research grants from Instituto de Salud Carlos III—Ministry of Economy and Innovation (Spain), CIBERER, Fundació La Marató, GBS-CIDP Foundation International, UCB and Grífols, received speaker or expert testimony honoraria from CSL Behring, Novartis, Sanofi-Genzyme, Merck, Annexon, Alnylam, Biogen, Janssen, Lundbeck, ArgenX, UCB, LFB, Octapharma and Roche, serves at Clinical Trial Steering Committee for Sanofi-Genzyme and Roche and is Principal Investigator for UCB’s CIDP01 trial. The other authors report no disclosures.