Employers must withhold and pay health insurance contributions at least from the minimal assessment base.
As soon as income from employment exceeds the minimum wage, health insurance premiums are to be paid at a rate of 13.5% of the reported gross wage of the employee; in such a case, the premium payments which must be paid for a parallel "side employment" need not be recalculated to be brought up to the level of the minimum assessment base (i.e., the level of the minimum wage).
If the employee does not reach the minimum assessment base in any of his or her employment relationships, then the one employer who has been authorized by the employee to do their payroll accounting for them shall recalculate the premium payments in the relevant higher amount (i.e., in an amount which reflects the minimum assessment base, whereas the minimum assessment base equals the minimum wage of CZK 11,000).
If the employee attains the minimum assessment base in their "primary employment relationship", but not in their side occupation, then the second employer must be presented with a certificate stating that the employee's health insurance contributions are already withheld by another employer, and that they are based on the minimum assessment base.
Based on this certificate, the second employer need not do the recalculation to reflect the minimum wage, but merely withholds and pays health insurance premiums based on the actual level of their employee's income.
Where dependent employment occurs in parallel to self-employed gainful occupation, the employee must present the employer with an affidavit to the effect that he or she (i.e., the employee) makes at least the minimum payments towards health insurance to which they are required under the law; in such a case, the employer may make "their" contribution payments based on the amount of the employee's actual income (derived from employment with this particular employer).
If an employer employs a foreigner from another EU member state who continues to draw a salary from employment in their country of permanent residence, then the foreign employee must present the employer with Form A1, so as to evidence the fact that they continue to be subject to the public health insurance scheme of the country in which they reside. In such a case, the employer need not make payments towards health insurance under Czech law. However, the employer ought to enquire whether they might not be obliged to make premium payments pursuant to the laws of the country in whose health insurance scheme the employee is enrolled).
Source: Act on general health insurance premiums