Note 1 - Organization and Basis of Presentation (Details Textual)								3 Months Ended	10 Months Ended	12 Months Ended			15 Months Ended	21 Months Ended
	Oct. 13, 2020 USD ($)	Jul. 13, 2020 USD ($) shares	Mar. 02, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Apr. 11, 2019 USD ($) $ / shares shares	Mar. 20, 2019 USD ($) $ / shares shares	Feb. 13, 2018 shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Jun. 08, 2020 $ / shares shares	Jun. 07, 2020 $ / shares shares
Business Combination, Stock Conversion, Shares Issued Per Common Share Of Surviving Entity (in shares)							0.22148
Stock Issued During Period, Shares, Reverse Stock Splits (in shares)							7,100,000
Exchange Ratio, Number of Shares Exchanged Per Share (in shares)							0.0201346
Stock Issued During Period, Shares, New Issues (in shares)						1,200,000
Gross Proceeds from Issuance of Common Stock | $						$ 3,000,000
Proceeds from Issuance of Common Stock | $						2,500,000
Payments of Stock Issuance Costs | $						$ 320,000
Payments for Aggregate Commissions to Underwriters | $					$ 750,000
Proceeds From Exercise of Pre-funded Warrants | $											$ 1,226,000
Common Stock, Par or Stated Value Per Share (in dollars per share) | $ / shares								$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001	$ 0.10
Common Stock, Shares Authorized (in shares)								150,000,000	150,000,000	150,000,000	150,000,000		150,000,000	150,000,000	150,000,000	100,000,000
Stock Issued During Period, Value, New Issues | $											$ 2,440,000
The April 2019 Offering [Member]
Stock Issued During Period, Shares, New Issues, Excluding Shares Sold to Underwriters (in shares)					925,455
Shares Issued, Price Per Share (in dollars per share) | $ / shares					$ 1.10
Proceeds from Issuance or Sale of Equity, Total | $					$ 9,300,000
Proceeds from Issuance or Sale of Equity, Net | $					$ 8,100,000
The September 2019 Offering [Member]
Stock Issued During Period, Shares, New Issues, Excluding Shares Sold to Underwriters (in shares)				26,124,828
Shares Issued, Price Per Share (in dollars per share) | $ / shares				$ 0.30
Pre-funded Warrant Price (in dollars per share) | $ / shares				0.20
Pre-funded Warrant, Accompanying Warrant, Amount Paid Per Share (in dollars per share) | $ / shares				$ 0.10
Payments for Aggregate Commissions to Underwriters | $				$ 713,000
Proceeds from Issuance or Sale of Equity, Total | $				8,700,000
Proceeds from Issuance or Sale of Equity, Net | $				$ 7,700,000
The March 2020 Offering [Member]
Stock Issued During Period, Shares, New Issues (in shares)			4,000,000
Payments of Stock Issuance Costs | $			$ 775,000
Proceeds from Issuance or Sale of Equity, Total | $			10,000,000
Proceeds from Issuance or Sale of Equity, Net | $			$ 9,200,000
ATM Program [Member]
Stock Issued During Period, Shares, New Issues (in shares)		12,503,806
Proceeds from Issuance of Common Stock | $		$ 97,000,000								$ 101,934,000
Stock Issued During Period, Value, New Issues | $		$ 100,000,000
Open Market Sale Agreement [Member]
Stock Issued During Period, Shares, New Issues (in shares)								692,651
Proceeds from Issuance of Common Stock | $								$ 4,900,000
Payments of Stock Issuance Costs | $	$ 300,000
Stock Issued During Period, Value, New Issues | $								$ 5,500,000
Open Market Sale Agreement, Maximum Aggregate Offering Price | $	$ 250,000,000
Open Market Sale Agreement, Sales Commission, Percentage	4.50%
Warrants in Connection with March 2019 Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)						84,000
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)						1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares						$ 3.125
Warrants and Rights Outstanding, Term (Year)						5 years
Warrants and Rights Outstanding | $						$ 100,000
Warrants in Connection With the April 2019 Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)					10,454,546
Warrants and Rights Outstanding | $					$ 333,000
Class of Warrant Right, Number of Pre-funded Warrants Issued During Period (in shares)					8,165,455
Pre-funded Warrant Price (in dollars per share) | $ / shares					$ 1
Pre-funded Warrant, Accompanying Warrant, Amount Paid Per Share (in dollars per share) | $ / shares					$ 0.10
Warrants in Connection With the April 2019 Offering [Member] | Over-Allotment Option [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)					1,363,636
Pre-funded Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)				1	1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares				$ 0.10	$ 0.10
Class of Warrant Right, Number of Pre-funded Warrants Issued During Period (in shares)				4,100,000
Proceeds From Exercise of Pre-funded Warrants | $													$ 400,000	$ 800,000
Class of Warrant or Right, Exercised During Period (in shares)													4,100,000	8,165,455
Common Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)				1	1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares				$ 0.30	$ 1.10
Warrants and Rights Outstanding, Term (Year)				5 years	5 years
Class of Warrant or Right, Exercised During Period (in shares)													30,661,667	10,228,580
Proceeds from Exercise of Common Stock Warrants | $													$ 9,200,000	$ 11,300,000
Underwriter Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)					1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares					$ 1.375
Class of Warrant or Right, Issued During Period (in shares)					636,364
Class of Warrant or Right, Period to Expiration (Year)					5 years
Class of Warrant or Right, Issued During Period, Fair Value | $					$ 333,000
Class of Warrant or Right, Exercised During Period (in shares)														524,433
Proceeds from Exercise of Common Stock Warrants | $														$ 700,000
Warrants in Connection with the September 2019 Offering [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				30,666,667
Warrants and Rights Outstanding | $				$ 497,000
Warrants in Connection with the September 2019 Offering [Member] | Over-Allotment Option [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)				4,000,000
Underwriter Warrants, September 2019 [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)				1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares				$ 0.375
Warrants and Rights Outstanding | $				$ 497,000
Class of Warrant or Right, Issued During Period (in shares)				2,115,738
Class of Warrant or Right, Period to Expiration (Year)				5 years
Class of Warrant or Right, Exercised During Period (in shares)													1,890,941
Proceeds from Exercise of Common Stock Warrants | $													$ 700,000
The March 2020 Offering Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares)			2,000,000
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)			1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares			$ 2.50
Warrants and Rights Outstanding, Term (Year)			5 years
Warrants and Rights Outstanding | $			$ 453,000
Class of Warrant or Right, Exercised During Period (in shares)									1,683,416
Proceeds from Exercise of Common Stock Warrants | $									$ 4,200,000
The March 2020 Offering Placement Agent Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right (in shares)			1
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares			$ 3.125
Warrants and Rights Outstanding | $			$ 453,000
Class of Warrant or Right, Issued During Period (in shares)			280,000
Class of Warrant or Right, Exercised During Period (in shares)									10,504
Private Vaxart's Stockholders, Warrantholders, and Optionholders [Member]
Ownership Percentage							51.00%
Aviragens Stockholderrs and Optionholders [Member]
Ownership Percentage							49.00%
Reverse Stock Split [Member]
Stockholders' Equity Note, Stock Split, Conversion Ratio							11