Effectiveness of an mRNA vaccine booster dose against SARS-CoV-2 infection and severe COVID-19 in persons aged ≥60 years and other high-risk groups during predominant circulation of the delta variant in Italy, 19 July to 12 December 2021

ABSTRACT

Background

Consolidated information on the effectiveness of COVID-19 booster vaccination in Europe are scarce.

Research design and methods

We assessed the effectiveness of a booster dose of an mRNA vaccine against any SARS-CoV-2 infection (symptomatic or asymptomatic) and severe COVID-19 (hospitalization or death) after over two months from administration among priority target groups (n = 18,524,568) during predominant circulation of the Delta variant in Italy (July–December 2021).

Results

Vaccine effectiveness (VE) against SARS-CoV-2 infection and, to a lesser extent, against severe COVID-19, among people ≥60 years and other high-risk groups (i.e. healthcare workers, residents in long-term-care facilities, and persons with comorbidities or immunocompromised), peaked in the time-interval 3–13 weeks (VE against infection = 67.2%, 95% confidence interval (CI): 62.5–71.3; VE against severe disease = 89.5%, 95% CI: 86.1–92.0) and then declined, waning 26 weeks after full primary vaccination (VE against infection = 12.2%, 95% CI: −4.7–26.4; VE against severe disease = 65.3%, 95% CI: 50.3–75.8). After 3–10 weeks from the administration of a booster dose, VE against infection and severe disease increased to 76.1% (95% CI: 70.4–80.7) and 93.0% (95% CI: 90.2–95.0), respectively.

Conclusions

These results support the ongoing vaccination campaign in Italy, where the administration of a booster dose four months after completion of primary vaccination is recommended.

KEYWORDS:

• SARS-CoV-2 infection
• COVID-19
• vaccine effectiveness
• booster dose
• delta variant
• Italy

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Ethics statement

This study, based on routinely collected data, was not submitted for approval to an ethical committee because the dissemination of COVID-19 surveillance data was authorized by the Italian Presidency of the Council of Ministers on 27 February 2020 (Ordinance no. 640). Because of the retrospective design and the large size of the population under study, in accordance with the Authorization n. 9 released by the Italian data protection authority on the 15th of December 2016, the individual informed consent was not requested for the conduction of this study.

Data sharing

Because of data sharing legal restrictions, the dataset including individual records cannot be made publicly available. However, aggregated data will be shared on reasonable request to the corresponding author (MF).

Acknowledgments

We would like to thank Gianpaolo Scalia Tomba for his useful suggestions and:

The members of the Italian Integrated Surveillance of COVID-19 study group and of the COVID-19 Vaccines Registry group: Antonino Bella, Alberto Mateo Urdiales, Martina Del Manso, Massimo Fabiani, Matteo Spuri, Chiara Sacco, Stefano Boros, Maria Cristina Rota, Antonietta Filia, Marco Bressi, Maria Fenicia Vescio, Daniele Petrone, Marco Tallon, Corrado Di Benedetto, Paola Stefanelli, Serena Battilomo, Valeria Proietti, Flavia Riccardo, Patrizio Pezzotti.

The regional representatives of the Italian Integrated Surveillance of Covid-19: Antonia Petrucci (Abruzzo); Michele La Bianca (Basilicata); Anna Domenica Mignuoli (Calabria); Pietro Buono (Campania); Erika Massimiliani (Emilia-Romagna); Fabio Barbone (Friuli Venezia Giulia); Francesco Vairo (Lazio); Camilla Sticchi (Liguria); Danilo Cereda (Lombardia); Lucia Di Furia (Marche); Francesco Sforza (Molise); Annamaria Bassot (P.A. Bolzano); Pier Paolo Benetollo (P.A. Trento); Chiara Pasqualini (Piemonte); Lucia Bisceglia (Puglia); Maria Antonietta Palmas (Sardegna); Salvatore Scondotto (Sicilia); Emanuela Balocchini (Toscana); Anna Tosti (Umbria); Mauro Ruffier (Valle D’Aosta); Filippo Da Re (Veneto);

The Regional representatives of the Italian Covid-19 Vaccines Registry: Camillo Odio (Abruzzo); Michele Recine (Basilicata); Innocenza Ruberto (Calabria); Salvatore Ascione e Massimo Bisogno (Campania); Gandolfo Miserendino, Massimiliano Navacchia (Emilia-Romagna); Beatrice Del Frate, Emanuela Cau (Friuli Venezia Giulia); Diego Baiocchi, Danilo Fusco (Lazio); Domenico Gallo (Liguria); Maria Rosa Marchetti (Lombardia); Liana Spazzafumo (Marche); Raffaele Malatesta (Molise); Antonio Fanolla (P.A. Bolzano); Diego Conforti, Carlo Trentini (P.A. Trento); Antonino Ruggeri (Piemonte); Concetta Ladalardo, Nehludoff Albano (Puglia); Marco Corona, Paolo Lombardi (Sardegna); Massimo Iacono (Sicilia); Paolo Bruno Angori, Andrea Belardinelli (Toscana); Milena Solfiti (Umbria); Stefano Fioraso (Valle D’Aosta); Chiara Poma, Nadia Raccanello (Veneto).

Author contributions

MF, MP, CM, SSA, AF, FDA, FR, AMU, FMI, and PPe designed the study. MS, MDM, MT, CS, MM, RDC, SBa, FV, MB, DP, and AB retrieved and prepared the data. MF, MP, MS, SSA, CS, VP, MM, and PPe carried out the analysis. MF, MP, CM, SSA, AF, FDA, FR, AMU, AS, PS, ATP, PPa, SBr, GR, FMI, MCR and PPe wrote the manuscript. All authors critically revised and approved the final version of the manuscript. All authors agree to be accountable for all aspects of the work.

Supplementary material

Supplemental data for this article can be accessed here