v3.25.2
Document and Entity Information - shares
6 Months Ended
Jun. 30, 2025
Aug. 08, 2025
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2025  
Document Transition Report false  
Entity File Number 001-32587  
Entity Registrant Name ALTIMMUNE, INC.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 20-2726770  
Entity Address, Address Line One 910 Clopper Road  
Entity Address, Address Line Two Suite 201S  
Entity Address, City or Town Gaithersburg  
Entity Address, State or Province MD  
Entity Address, Postal Zip Code 20878  
City Area Code 240  
Local Phone Number 654-1450  
Title of 12(b) Security Common stock, par value $0.0001 per share  
Trading Symbol ALT  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   88,257,253
Entity Central Index Key 0001326190  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Amendment Flag false