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The impact of the COVID-19 pandemic in sport has been the subject of numerous studies over the past two years. However, knowledge about the direct impact of COVID-19 infection on the performance of athletes is limited, and the importance of studies on this topic is crucial during the current pandemic era. This study aimed to evaluate the changes in the match running performance (MRP) of professional soccer players that occurred as a result of COVID-19 infection after fulfilling all of the prerequisites for a safe return to play (RTP). The participants were 47 professional soccer players from a team which competed in first Croatian division (21.6 years old on average) during the 2020/21 season. The total sample was divided into two subgroups based on the results of a PCR test for COVID-19, where 31 players tested positive (infected) and 16 tested negative. We observed the PCR test results (positive vs. negative PCR), the number of days needed to return to the team, number of days needed to RTP after quarantine and isolation, and MRP (10 variables measured by a global positioning system). The number of days where the infected players were not included in the team ranged from 7 to 51 (Median: 12). Significant pre- to post-COVID differences in MRP for infected players were only found for high-intensity accelerations and high-intensity decelerations (t-test = 2.11 and 2.13, respectively; p < 0.05, moderate effect size differences), with poorer performance in the post-COVID period. Since a decrease of the MRP as a result of COVID-19 infection was only noted in two variables, we can highlight appropriateness of the applied RTP. However, further adaptations and improvements of the RTP are needed with regard to high-intensity activities.

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There are limited data describing clinical patterns and match running performance (MRP) among players with COVID-19 infection before and after infection, particularly related to different predominant SARS-CoV-2 variants, as well as in comparison to uninfected players. This observational study was conducted during two consecutive soccer seasons in one professional club in Split, Croatia. There were four clusters of mild, self-limited, or asymptomatic infection characterised by low adherence to preventive measures. Infected players had significantly more symptoms (t-test = 3.24; p = 0.002), a longer period of physical inactivity (χ2 = 10.000; p = 0.006) and a longer period of self-assessment for achieving full fitness (χ2 = 6.744; p = 0.034) in the 2020–2021 season (Wuhan wild strain and Alpha variant) than in the 2021–2022 season (Omicron variant). It was also found that, despite the milder clinical presentation of the infection in the 2021–2022 season, the players had significantly more abnormal laboratory findings (χ2 = 9.069240; p = 0.002), although without clinical significance at the time of the study. As for the MRP, player performance in the 2021–2022 season was not negatively affected by the Omicron variant, while there was an improvement in MRP in scores for a sample of all players. The RTP protocol was correctly applied because it helped the athletes to recover their pre-infection physical capacities relatively quickly. This study advances the understanding that an optimally and individually planned RTP protocol is crucial for the MRP of infected players. Future research needs to replicate the findings of abnormal laboratory results and extend the study focusing on their potential long-term clinical significance.

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