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Equity Incentive Plans, Employee Stock Purchase Plan and Stock-Based Compensation - Additional Information (Details) - USD ($) |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 26, 2018 | Sep. 25, 2018 | Aug. 31, 2019 | Feb. 28, 2017 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019 | Jan. 01, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Reserved common stock |  |  |  |  | 8,473,259 |  | 8,473,259 |  | 7,357,234 |
Aggregate intrinsic value of stock options exercised |  |  |  |  | $ 48,000 | $ 63,000 | $ 129,000 | $ 119,000 |  |
Weighted-average estimated grant-date fair value of employee stock options granted |  |  |  |  | $ 5.69 | $ 4.88 | $ 5.09 | $ 6.74 |  |
Share-based compensation arrangement by share-based payment award options, shares granted |  |  |  |  |  |  | 266,724 |  | 266,724 |
Stock-based compensation expense |  |  |  |  | $ 3,112,000 | $ 2,880,000 | $ 8,830,000 | $ 7,629,000 |  |
Liability attributable to call option plan |  |  |  |  | $ 3,400,000 |  | $ 3,400,000 |  | $ 76,000 |
Restricted Stock Units |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Restricted common stock granted |  |  |  |  |  |  | 505,250 |  |  |
Shares vesting period |  |  |  |  |  |  | 4 years |  |  |
Shares vesting date |  |  |  |  |  |  | 2024-08 |  |  |
2018 Equity Incentive Plan |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Reserved common stock |  | 2,300,000 |  |  |  |  |  |  |  |
Annual increase period of common stock reserved for issuance |  | 10 years |  |  |  |  |  |  |  |
Maximum number of shares issuable |  |  |  |  |  |  |  |  |  | 1,154,948
Shares available for grant |  |  |  |  | 1,918,224 |  | 1,918,224 |  |  |
2018 Equity Incentive Plan | Restricted Stock Units |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Total unrecognized compensation cost related to unvested granted |  |  |  |  | $ 5,400,000 |  | $ 5,400,000 |  |  |
Remaining unrecognized compensation cost expected to be recognized over weighted-average period |  |  |  |  |  |  | 2 years 8 months 12 days |  |  |
2018 Equity Incentive Plan | Employee Stock Options |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Total unrecognized compensation cost related to unvested granted |  |  |  |  | 18,500,000 |  | $ 18,500,000 |  |  |
Remaining unrecognized compensation cost expected to be recognized over weighted-average period |  |  |  |  |  |  | 2 years 4 months 24 days |  |  |
2018 Equity Incentive Plan | Employee Stock Purchase Plan |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Unrecognized stock-based compensation expense |  |  |  |  | $ 300,000 |  | $ 300,000 |  |  |
2018 Equity Incentive Plan | Maximum |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Increase in stock reserved for issuance as percentage of capital stock outstanding on last day of preceding year |  | 5.00% |  |  |  |  |  |  |  |
2018 Employee Stock Purchase Plan |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Reserved common stock | 230,000 |  |  |  |  |  |  |  |  |
Annual increase period of common stock reserved for issuance | 10 years |  |  |  |  |  |  |  |  |
Maximum number of shares issuable | 2,300,000 |  |  |  | 361,539 |  | 361,539 |  |  |
Shares available for grant |  |  |  |  | 327,931 |  | 327,931 |  |  |
Shares available for grant, increase |  |  |  |  |  |  |  |  |  | 230,989
2018 Employee Stock Purchase Plan | Maximum |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Increase in stock reserved for issuance as percentage of capital stock outstanding on last day of preceding year | 1.00% |  |  |  |  |  |  |  |  |
Call Option Plan |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense |  |  |  |  | $ 90,000 | 24,000 | $ 158,000 | 59,000 |  |
Other expense net |  |  |  |  | $ 1,100,000 | $ 0 | $ 3,100,000 | $ 0 |  |
Call Option Plan | Option Granted in February 2017 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award options, shares granted |  |  |  | 248,944 |  |  | 248,944 |  |  |
Call options expiration period |  |  |  | 1 year |  |  |  |  |  |
Share-based compensation arrangement by share based payment award options, exercise price per share |  |  |  |  |  |  | $ 1.28 |  |  |
Call Option Plan | Option Granted in February 2017 | January 1 , 2017 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Call options vesting percentage |  |  |  | 25.00% |  |  |  |  |  |
Call Option Plan | Option Granted in February 2017 | January 1 , 2018 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Call options vesting percentage |  |  |  | 25.00% |  |  |  |  |  |
Call Option Plan | Option Granted in February 2017 | January 1 , 2019 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Call options vesting percentage |  |  |  | 25.00% |  |  |  |  |  |
Call Option Plan | Option Granted in February 2017 | January 1 , 2020 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Call options vesting percentage |  |  |  | 25.00% |  |  |  |  |  |
Call Option Plan | Option Granted in August 2019 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award options, shares granted |  |  | 17,780 |  |  |  | 17,780 |  |  |
Call options expiration period |  |  | 1 year |  |  |  |  |  |  |
Share-based compensation arrangement by share based payment award options, exercise price per share |  |  |  |  |  |  | $ 2.04 |  |  |
Call Option Plan | Option Granted in August 2019 | January 1 , 2019 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Call options vesting percentage |  |  | 25.00% |  |  |  |  |  |  |
Call Option Plan | Option Granted in August 2019 | January 1 , 2020 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Call options vesting percentage |  |  | 25.00% |  |  |  |  |  |  |
Call Option Plan | Option Granted in August 2019 | January 1 , 2021 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Call options vesting percentage |  |  | 25.00% |  |  |  |  |  |  |
Call Option Plan | Option Granted in August 2019 | January 1 , 2022 |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Call options vesting percentage |  |  | 25.00% |  |  |  |  |  |  |
Call Option Plan | Vaxcyte, Inc. |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |
Reserved common stock |  |  |  |  | 266,724 |  | 266,724 |  |  |

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