ISS is both a reward and an incentive. ISS funding is flexible cash which countries can use as they choose to improve immunisation performance.
GAVI provides support in two distinct phases:
1) An investment phase during the first and second years after approval: this support is considered an “investment” in the effort needed to raise coverage and meet planned immunisation targets.
2) A reward phase from the third year after approval to the year after the end of the comprehensive Multi-Year Plan (cMYP): this support is considered as a “reward”, or an incentive and is calculated according to country achievements in surpassing previous year targets. Continuation of the reward depends on strict performance monitoring.
Countries that are reapplying for ISS will not receive a second investment but will continue to receive rewards.
GAVI does not prescribe conditions for the use of these funds but imposes strict performance requirements and relies on governments and inter-agency coordinating committees to set goals and monitor progress. In countries implementing a sector-wide approach (SWAp), GAVI funds may contribute to the “common fund basket” used by the SWAp to finance primary health care.
To calculate the financial support during the investment and reward phases of support, GAVI uses a performance indicator and a baseline appropriate to the respective year.
As an indicator of prospective performance in the investment phase, GAVI uses the number of additional children less than one year old targeted to receive the third dose of DTP vaccine in the first year following approval of GAVI support, compared to the initial baseline.
For the initial baseline, GAVI uses the number of children less than one year old who have received the full three-dose series of DTP in the year preceding GAVI’s approval of the country proposal.
GAVI calculates the total investment by multiplying the prospective performance indicator by US$20. 
This total investment is disbursed in three portions:
(25% of the investment);
As a performance indicator in the reward phase, GAVI uses the number of additional children less than one year old who have received DTP3, compared to the baseline.
The baseline for the first reward is the number of additional children who were targeted to receive DTP3in the first year after approval of GAVI support.
The baseline for subsequent rewards is the number of additional children who were reported to have received DTP3 in the previous year, or, if this is lower than the initial target, compared to the target set.
GAVI calculates the reward each year by multiplying the performance indicator by US$20. 
Rewards are paid each year, starting in the third year after approval, subject to submission of a satisfactory annual report. Countries have to conduct a Data Quality Audit (DQA) to show that their immunisation reporting system is robust.