question: BACKGROUND: The surge and rapid global spreading of SARS-CoV-2 infection has deeply challenged health services and disrupted social and economic activities worldwide. In Spain, the arrival of the first pandemic wave occurred in mid-March 2020 and lasted for 3 months, being abated with home confinement and strict lockdown. After relaxing measures during summer, a second wave began in mid-September and extended until Christmas 2020. METHODS: We compared the main features of SARS-CoV-2 infections during the two pandemic waves. We examined the information collected with rapid diagnostic tests and PCR at one university clinic in Madrid, epicenter of the pandemic in Spain. RESULTS: A total of 1569 individuals (968 during the 1(st) wave and 601 during the 2(nd) wave) were tested for SARS-CoV-2 specific antibodies using finger prick capillary blood. In addition, during the second wave, 346 persons were tested for SARS-CoV-2 specific antigens using either oral swab or saliva. The overall seroprevalence of first-time tested persons was 12.6% during the 1(st) wave and 7.7% during the 2(nd) wave (p < 0.01). Seroconversions and seroreversions were noticed within 6 months at low rates, both below 5%. Positive SARS-CoV-2 antigen during the 2(nd) wave was recognized in 3.5% of tested individuals, being two cases considered as reinfections. Severe clinical symptoms occurred in a greater proportion during the first wave compared to the second wave (27.8% vs 10.6%, respectively; p = 0.03). CONCLUSION: The cumulative seroprevalence of SARS-CoV-2 antibodies in Madrid at the end of year 2020 was around 20%. Seroreversions within six months occurred in 4%. Seroconversions and reinfections during the second wave were clinically less severe than during the 1(st) wave. Hypothetically, promotion of wider face masking, outdoor activities, and gathering restrictions during the 2nd wave most likely contributed to reduced exposures to large size inoculum.