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Net Loss per Share | 12 Months Ended
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Dec. 31, 2021
Earnings Per Share [Abstract] |
Net Loss per Share | 16. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. was calculated as follows (in thousands, except share and per share amounts):
|  | Year Ended December 31, |
|  |  | 2021 |  |  |  | 2020 |
Numerator: |  |  |  |  |  |
Net loss |  | $ | (89,756 | ) |  | $ | (78,280 | )
Deemed Dividend |  |  | — |  |  |  | (549 | )
Net loss attributable to common stockholders |  | $ | (89,756 | ) |  | $ | (78,829 | )
Denominator: |  |  |  |  |  |
Weighted average common shares outstanding, basic and diluted |  |  | 30,895,756 |  |  |  | 22,386,122 |
Net loss per share, basic and diluted |  | $ | (2.91 | ) |  | $ | (3.52 | )
The net loss applicable to common stockholders for the year ended December 31, 2020 did not equal net loss due to the accretion of the beneficial conversion feature of Series C Preferred Stock in the amount of $0.5 million. The beneficial conversion feature was initially recorded as a discount on the Series C Preferred Stock with a corresponding amount recorded to Additional Paid- in Capital. The discount on the Series C Preferred Stock was then immediately written off as a deemed dividend as the Series C Preferred Stock does not have a stated redemption date and is immediately convertible at the option of the holder. The Company excluded potentially dilutive securities from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:
|  | Year Ended December 31, |
|  |  | 2021 |  |  |  | 2020 |
Options to purchase common stock |  |  | 4,890,716 |  |  |  | 3,682,233 |
Unvested restricted stock units |  |  | 513,690 |  |  |  | 30,561 |
Series A convertible preferred stock (as converted to common shares) |  |  | — |  |  |  | — |
Series B convertible preferred stock (as converted to common shares) |  |  | 938,000 |  |  |  | 1,000,000 |
Series C convertible preferred stock (as converted to common shares) |  |  | 2,214,000 |  |  |  | 2,287,000 |
Series D convertible preferred stock (as converted to common shares) |  |  | 3,215,000 |  |  |  | 3,215,000 |
Total |  |  | 11,771,406 |  |  |  | 10,214,794 |

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