Segment information - Significant Segment Expenses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Indirect research and development expenses:
Total research and development expenses	$ 7,953	$ 11,662	$ 16,243	$ 22,787
Research and Development of Immunotherapies and Vaccines
Direct research and development expenses:
Total direct research and development expenses	4,710	6,438	8,202	12,277
Change in direct research and development expenses	(1,728)		(4,075)
Indirect research and development expenses:
Total indirect research and development expenses	3,243	5,224	8,041	10,510
Change In indirect research and development expenses	(1,981)		(2,469)
Total research and development expenses	7,953	11,662	16,243	22,787
Change in research and development expenses	(3,709)		(6,544)
Research and Development of Immunotherapies and Vaccines \| Personnel-related (including share-based compensation)
Indirect research and development expenses:
Total indirect research and development expenses	2,450	4,763	6,394	9,097
Change In indirect research and development expenses	(2,313)		(2,703)
Research and Development of Immunotherapies and Vaccines \| Facility related
Indirect research and development expenses:
Total indirect research and development expenses	350	342	685	732
Change In indirect research and development expenses	8		(47)
Research and Development of Immunotherapies and Vaccines \| Other indirect costs
Indirect research and development expenses:
Total indirect research and development expenses	443	119	962	681
Change In indirect research and development expenses	324		281
Research and Development of Immunotherapies and Vaccines \| VTP-1000 Celiac
Direct research and development expenses:
Total direct research and development expenses	1,782	1,371	2,764	2,744
Change in direct research and development expenses	411		20
Research and Development of Immunotherapies and Vaccines \| VTP-300 HBV
Direct research and development expenses:
Total direct research and development expenses	1,837	3,034	3,187	4,947
Change in direct research and development expenses	(1,197)		(1,760)
Research and Development of Immunotherapies and Vaccines \| Other clinical programs
Direct research and development expenses:
Total direct research and development expenses	642	1,125	1,383	2,893
Change in direct research and development expenses	(483)		(1,510)
Research and Development of Immunotherapies and Vaccines \| Other pre-clinical programs
Direct research and development expenses:
Total direct research and development expenses	449	908	868	1,693
Change in direct research and development expenses	(459)		(825)
Research and Development of Immunotherapies and Vaccines \| VTP-500 MERS \| Personnel-related (including share-based compensation)
Indirect research and development expenses:
Total indirect research and development expenses	$ 100	$ 200	$ 100	$ 300