Citywire A-rated Woodford went from below a reportable holding in the business to owning 19% in one purchase, or 17.45 million shares worth £9.07 million at a share price of 52p.
The manager - who has offered the best long-term risk-adjusted return of any UK investor - left Invesco Perpetual, his employer of more than 25 years, earlier this year to launch his own business.
Shares in Synairgen briefly spiked more than 40% in June following the announcement of the deal with AstraZeneca, before deflating as the company placed 12.2 million shares to fund development.
AstraZeneca will pay the company an up-front fee of $7.25 million with the remainder subject to expectations, in addition to royalties. Broker finnCap has a 108p target price on the business.