Acknowledgements We thank all participants of the study and we acknowledge the work of all individuals at the Oxford Vaccine Group, the Turku University group, the Dutch Spaarne Hospital and RIVM who have undertaken the work of the BERT study including the administrative staff, play assistants, clinical research nurses, clinical research fellows, laboratory staff and members of the IT team. In particular, we acknowledge Jacqueline Zonneveld and Greetje van Asselts at the Spaarne Gasthuis Hospital, Hoofddorp, the Netherlands for their help in the management of clinical data and performing home visits. We are also grateful to Inge Pronk and Eleonora Lambert, who both work at the Centre for Infectious Disease Control, RIVM for their assistance in clinical trial management. PERISCOPE has received funding from the Innovative Medicines Initiative 2 Joint Undertaking under grant agreement No 115910. This Joint Undertaking receives support from the European Union's Horizon 2020 research and innovation programme and EFPIA and BMGF. DFK receives salary for support from the NIHR Oxford Biomedical Research Centre.