2.1. Study Design and Participants

This prospective longitudinal study of two cohorts was conducted at the Instituto Nacional de Cancerología in Mexico (INCan) from September 2021 to December 2021. This study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the ethics and research committees of INCan (approval number: 022/006/ICI; CEI/1586/21). Informed consent was obtained from all participants.

During this period, participants were vaccinated against SARS-CoV-2 according to the national vaccination program applied to the Mexican population. Participants received mRNA vaccines, such as BNT162b29 (Pfizer-BioNTech, NY, USA) and Johnson & Johnson’s Janssen, viral vector vaccines including ChAdOx1 (Oxford/AstraZeneca, Cambridge, UK), Gam-COVID-Vac (Sputnik V) and Ad5-nCoV-S (CanSino), or inactivated virus vaccines like Sinovac-CoronaVac in one or two doses according to the pre-established scheme. Vaccines were administered according to those available at the local healthcare centers.

The first cohort comprised LC patients over 18 years recruited at their routine visit to the oncology service. All cancer patients received CTX, immunotherapy, tyrosine kinase inhibitors (TKIs) or concomitant treatments when having the first and second doses of COVID-19 vaccines. The second cohort consisted of healthy individuals without a known cancer diagnosis or other disorders leading to immunodeficiency.

All participants were asked for vaccination status against COVID-19 and previous history of SARS-CoV-2 infection. Vaccine adverse effects were documented according to the Mexican official standard for epidemiological surveillance, which grades the severity of symptoms in four groups in increasing order.

Clinical data regarding cancer treatment were obtained from medical records. Blood samples were collected 30 days after the second dose of the COVID-19 vaccine to determine antibodies against SARS-CoV-2 spike protein (S protein), CD19⁺ B-cells, antibody-secreting B cells (ASBC), CD27(−) B cells, and memory B lymphocytes.

We excluded participants with clinical suspicion or microbiological evidence of active COVID-19 infection.