Coronaviruses are a large family of viruses which assume the prefix “corona,” from the many crown-like spikes on their surface. The novel SARS-CoV-2 coronavirus which sustains the current pandemic outbreak appears as a completely new pathogen, sharing modest similarities with SARS-CoV-1 and MERS-CoV. It was firstly identified in Wuhan China in December 2019 and since then it has now spread worldwide. SARS-CoV-2 contains four structural proteins distributed in the form of one internal protein (nucleocapsid) and three external proteins (envelope, membrane, and spike). The nucleocapsid protein is inside the virus, associated with viral RNA, whilst the spike protein contains the receptor-binding domain (RBD) to natural receptors on host cells surfaces, mainly angiotensin-converting enzyme 2 (ACE 2) .
Due to the ongoing COVID-19 pandemic, vaccination continues to be critically important because it is regarded as safe and effective means to prevent disease and reduce virulence [2, 3]. The current COVID-19 vaccines are not specifically designed to avoid contracting viral infection since the main target of vaccination now is focused on eliciting an immune response against the virus and thus lowering its pathogenicity (i.e. protecting against serious illness or death) [4, 5]. COVID-19 vaccines have been developed following equivalent legal requirements for efficacy and safety as for all other vaccines. In the European Union, before being made commercially available, any COVID-19 vaccine candidate needs to meet the severe requirements and evidence thresholds of the European Medicines Agency (EMA) . What is different in such case is that development and approval have been much faster, due to the public-health emergency caused by this new coronavirus. COVID-19 vaccination campaign across the European Union started on 27 December 2020, and it is still in progress. The European Commission has so far given the provisional marketing approval for 4 vaccines (Pfizer/BioNTech, Moderna, Oxford/AstraZeneca and Janssen Pharmaceuticals) .
In order to boost the vaccination rate some different strategies have been proposed such as splitting the doses, delaying the second dose, heterologous vaccination, delay of vaccination in seropositive recipients . In addition, a practical problem with some types of vaccines is the multi-vial dose format and the need of preparation before administration, whereby its contribution to vaccine wastage (sum of vaccines discarded, damaged and lost) must be reduced. Finally, one more controversial subject is the anti-SARS-CoV-2 serological monitoring of COVID-19 vaccinated population .
For vaccines that require two injections, healthcare organizations have typically recommended the second shot to be the same as the former. Following safety issues, mostly related to cases of atypical venous thrombosis, some European countries have decided to stop the use of the adenovirus-based Oxford/AstraZeneca vaccine. Consequently, millions of people were unable to receive a second dose of such vaccine, remaining only partially vaccinated. To resolve this matter, mix-and-match vaccine studies have been planned aimed to investigate the safety and immune response in people receiving two different types of COVID-19 vaccine [10–11]. Some preliminary studies carried out in different countries suggest that combining different vaccines induces potent immune response [12–15], but some safety concerns remain .
The BNT162b2 mRNA COVID-19 vaccine has been designed to be given in two 30 µg doses, 21 days apart . Millions of doses have been now administered around the world, but in some cases vaccine overdose has occurred, mostly due to human errors. A summary of the cases reported worldwide is as follows: Israel-Jerusalem (3); Germany-Stralsund (8); Singapore-Singapore (1); Australia-Brisbane (2); USA-Ford Madison (77); Italy-Massa (1); Spain-Seville (8). In addition, the company informed in the vaccine brochure that during a clinical trial 52 participants received an erroneous dilution equivalent to a nearly double dose of vaccine (58 µg) . In these cases, the mistake was due to the use of “multi-dose” vials  which are useful in a pandemic situation because they allow cheaper and more efficient distribution. However, multi-dose vaccines, mainly when the vaccine needs to be reconstituted before injection, are more prone to administration errors. Specifically, each single vial of the Pfizer/BioNTech vaccine contains multiple doses (between five and six, in general). The active component of the vaccine (0.45 mL) must be diluted using 0.9% sodium chloride (1.8 mL). As a consequence, dosing errors may be caused by omitting the mandatory dilution by healthcare operators, so that a concentrated (higher dosage) product could be mistakenly injected.
Therefore, the main purpose of this study was to report the anti-SARS-CoV-2 RBD immunoglobulin class G (Ig G) antibody response in recipients of erroneous Pfizer/BioNTech vaccination schedule (overdosage)