Isolation of close contact people and negative test certification are used to manage the spread of new coronavirus infections worldwide. These effectively prevent the spread of infection in advance, but they can lead to a decline in socio-economic activity. Thus, the present study quantified the extent to which isolation and negative test certification respectively reduce the risk of infection. To this end, a discrete-time SEIR model was used as the infectious disease model, and equations for calculating the conditional probability of non-infection status given negative test results on two different days were derived. Then the respective non-infection probabilities with two negative PCR test results, and with one negative PCR test result and one antigen test result, were quantified. By substituting initial parameters of the SEIR model into these probabilities, the present study revealed the following: (1) isolating close contact individuals can reduce by\documentclass[12pt]{minimal}
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				\begin{document}$$80\%$$\end{document}80%the risk of infection during the first 5 days, but five more days are needed to reduce the risk\documentclass[12pt]{minimal}
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				\begin{document}$$10\%$$\end{document}10%more, and seven more days to reduce the risk\documentclass[12pt]{minimal}
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				\begin{document}$$20\%$$\end{document}20%more; and (2) if an individual with a negative PCR test result has a negative antigen test result the next day, then his or her infection probability is between 0.6 and\documentclass[12pt]{minimal}
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				\begin{document}$$0.7\%$$\end{document}0.7%. Our results show that 5-day isolation has a proportionally greater effect on risk reduction, compared to longer isolation; and thus, if an isolation period of longer than 5 days is contemplated, both the risk reduction and the negative effects from such increased isolation should be considered. Regarding negative test certification, our results provide those in managerial positions, who must decide whether to accept the risk and hold mass-gathering events, with quantitative information that may be useful in their decision-making.