The Coronavirus Disease 2019 (COVID-19) pandemic has claimed over 5 million lives globally. Fortunately, a substantial and growing number of SARS-CoV-2 vaccines with very high efficacy have been developed, manufactured, and rapidly approved. Novel mRNA vaccines such as the BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) have reported a stunning >94% efficacy against COVID-19. However, global access has not been equitable, with many low- and middle-income countries having no vaccine access or access under emergency use mainly to traditional inactivated SARS-CoV2-2 vaccines such as BBIBP-CorV (Sinopharm Beijing), CoronaVac (Sinovac) and BBV152 (Bharat Biotech). Emerging studies have shown that lower concentrations of neutralizing antibodies (Nab) are attained after CoronaVac than after a mRNA-based vaccine in healthy individuals. This difference seems to be more pronounced in immunocompromised patients who are at higher risk of severe COVID-19 and death from COVID-19. As such several countries including United States, Israel and Chile have recommended a third vaccine dose for this vulnerable population.

In this observational study we will explore the humoral response to the BNT162b2 vaccine in patients who received two previous doses of the inactivated vaccine Coronavac or two doses of BNT162b2.

Inclusion Criteria:

• Vaccination with two doses of Coronavac vaccine or BNT162b2 vaccine
• Eastern Cooperative Oncology group performance status < 3

Exclusion Criteria:

• Previous SARS-CoV-2 infection
• Vaccination with booster vaccine more than 12 weeks before informed consent
• Intravenous inmunoglobulin therapy 60 days before informed consent
• Any condition that could interfere with the paticipant´s participation during the study in the opinion of the treating investigator.

Cancer patients receiving their care at the Red de Salud UC Christus in Santiago Chile will be invited