The coronavirus disease COVID-19 is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Not only can this virus induce severe respiratory disease, but it can also induce multiple histopathological changes in multiple systems and organs, including the kidney (1), brain (2), and liver (3). It utilizes angiotensin-converting enzyme 2 (ACE2) for cell entry (4). ACE2 receptor expression has been identified in the genitourinary organs and the testis (5–8), so the testis (9) and ovary (10) may also be potential target organs for virus infection. In the initial stages of the pandemic, the American Society for Reproductive Medicine (ASRM) issued guidance recommending the suspension of most assisted reproductive technology (ART) treatments, except in the most urgent cases. This recommendation was in line with the guidance provided by the European Society for Human Reproduction and Embryology (11). As a consequence, there has been a significant decline in the number of patients attending infertility clinics over the past three years. Additionally, the majority of patients undergoing in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) have opted to either cancel their cycles or freeze oocytes or embryos (12, 13). With the easing of nationwide restrictions relating to coronavirus disease 2019 (COVID-19) since December 2022, reproductive centers are likely to encounter an increasing number of infected patients. The region where our center is situated experienced a concentrated outbreak of COVID-19 between December 2022 and January 2023, providing a reliable opportunity for us to gather pertinent data.

Most previous studies have primarily concentrated on the impact of COVID-19 infection on human reproductive function, particularly focusing on analysis of male semen and the potential detection of COVID-19 mRNA or antibodies in semen (14, 15), follicular fluid, oocytes, endometrial tissue (12, 16, 17), and cervicovaginal secretions (18) of infected patients. In contrast, there is a paucity of literature addressing the specific effects of COVID-19 infection on the pregnancy outcomes of IVF/ICSI procedures. Additionally, two studies have reached opposite conclusions regarding the impact of COVID-19 infection on embryos. Chen et al. report that COVID-19 does not adversely affect oocyte quality or embryo development (19). In contrast, another study posits that previous SARS-CoV-2 infection might influence the developmental potential of embryos (20). Although recent research (21–23) has reported no negative impact of COVID-19 infection on the clinical outcomes of ART treatments, these studies may have limitations due to their small case group sample sizes and the lack of consideration for the impact of the woman’s age on clinical outcomes.

In light of the above, despite the effective control of COVID-19, the pandemic has not been completely eradicated. Sporadic cases continue to occur, and instances of reinfection have been reported. Consequently, studies investigating the impact of COVID-19 on the clinical outcomes of women undergoing ART cycles remain of critical importance.

For the general female population, a history of COVID-19 infection may not adversely affect pregnancy outcomes. However, when focusing on different age groups, the study found that for women over the age of 38, the likelihood of clinical pregnancy significantly decreases with each additional year of age.

Serum levels of FSH, AMH, and basal AFC on days 2–3 of the menstrual period are the three most frequently used and effective markers for ovarian reserve. Kolanska et al. found that mild COVID-19 infection does not alter the ovarian reserve in women treated with ART (27), and similar conclusions were reached in a study by Kahyaoglu et al. (28). In our study, we observed that basal follicle-stimulating hormone (FSH) levels were lower and AFC was higher in the COVID-19 group compared to the non-COVID-19 group, while AMH levels were similar between both groups. This suggests that the data from the population examined during this period do not support the conclusion that COVID-19 infection impacts ovarian reserve function. Furthermore, a higher proportion of patients in the COVID-19 group underwent antagonist protocols; this group was also associated with lower Gn dosage and shorter Gn durations than the non-COVID-19 group. This was likely due to the preference for fast entry cycles and short treatment courses during the pandemic to minimize hospital visits, reduce the risk of nosocomial infection, and improve treatment efficiency. Additionally, a significant decrease in the number of blastocyst formations was observed in the COVID-19 group compared to the non-COVID-19 group; this is in alignment with the findings of Jin Lei et al. (22), who also reported a decrease in blastocyst formation rates following COVID-19 infection. This may be related to the significant co-expression of ACE 2 and TMPRSS2 in the trophoblast ectoderm of late blastocysts, which are more sensitive to SARS-CoV-2 (29). Due to the indeterminate nature of the impact on pregnancy outcome after COVID-19 infection, patients in the COVID-19 group were more likely to be selected for whole-embryo freezing. After adjustment for confounding factors, including the ages of the couple, type and durations of infertility, causes of infertility, and controlled ovarian stimulation (COS) protocol, logistic regression analysis revealed that prior COVID-19 infection did not significantly affect clinical pregnancy rate. These findings are in line with several previous studies that have drawn similar conclusions (30–32).

Age of the woman has a significant impact on embryo quality and pregnancy outcome among patients undergoing assisted conception. In our study, the results of curve-fitting indicated that clinical pregnancy rates were lower among women over the age of 38 in both groups, with the difference being more significant in the COVID-19 group. Under a threshold effect model, the results showed that for women aged over 38 years, the likelihood of clinical pregnancy declined by 53% for every additional year in the COVID-19 group. In order to explore why the clinical outcomes were poorer among older women infected with COVID-19, we compared the data from couples in which the woman was over the age of 38 between the two groups, and found that the available embryo rate, the rate of available embryos per egg, the high-quality embryo rate, the rate of high-quality embryos per egg, the number of blastocysts formed, the blastocyst formation rate, semen parameters after recovery, and sperm concentration on the day of oocyte retrieval were all lower in the COVID-19 group (the results of analysis of these variables are given in Supplementary Table 4 ). Some other studies also have explored potential reasons for the decline in fertility caused by COVID-19. Several studies have reported that oxidative stress plays an important role in COVID-19 infection at the molecular level (33, 34), and the antioxidant system and the accumulation of reactive oxygen species are among the possible reasons for poor pregnancy outcomes. Increased oxidative stress activates the pathogenic mechanism of female fertility (35), alters oocyte epigenetics (36), and ultimately has a negative impact on oocyte quality (37). These two factors may constitute one possible explanation for the more significant decrease in clinical pregnancy rate with advanced age after COVID-19 infection. The challenge of poor pregnancy outcomes in the population of older couples undergoing ART is well-recognized among clinicians, and our study suggests that COVID-19 infection exacerbates these outcomes in this demographic. Consequently, individualized treatment approaches, tailored to the specific needs of this population, are warranted.

Does COVID-19 actually affect gametes? Various studies have provided differing answers. Youngster (19) suggests that COVID-19 infection might have a long-term negative effect on oocyte yield when retrieval occurs more than 180 days after infection. However, a study by Dolgushina presents an opposing view, finding that the parameters of oogenesis and embryogenesis, as well as pregnancy and childbirth rates, did not differ between groups with time intervals of ≤180 days or >180 days (38). In our study, we analyzed the impact of the time interval from recovery to oocyte retrieval on clinical pregnancy rate and found no significant effect. This might be attributed to our study’s time interval range of 24–167 days, which did not extend to 180 days. However, our result was consistent with that of a study by Huang, Jialyu et al. (32), which indicated that prior SARS-CoV-2 infection in females did not adversely affect subsequent IVF treatment, regardless of the time interval following infection. Regarding sperm, another study (39) involving 120 COVID-19 infected subjects found that sperm parameters gradually improved, during convalescence after documented COVID-19 infection from testing an average 53 days after a positive SARS-CoV-2 nasopharyngeal PCR test, suggesting recovery over time following the viral infection.

This study did not include patients undergoing FET, but there are related studies offering insights. Aizer, Adva et al. found that COVID-19 infection did not affect implantation rates or clinical or ongoing pregnancy rates in subsequent FET cycles (31). However, research by Youngster et al. (40) indicated that the clinical pregnancy rate after FET was significantly lower in women infected less than 60 days prior compared to non-infected patients, although there was no significant difference for patients infected more than 60 days prior. This raises the question of whether appropriately delaying pregnancy to allow for normalization of semen parameters, or opting for egg freezing after retrieval or transfer after resuscitation in whole-embryo freezing cycles, could be effective strategies. Additionally, the impact of increased age on clinical outcomes due to delayed assisted reproduction must be considered. Therefore, the optimal time interval before pregnancy and the effectiveness of these methods in improving pregnancy outcomes require further research.

Although larger in terms of sample size than previous research on COVID-19 and pregnancy outcomes, this study has several limitations. Firstly, the potential for infinite statistical differences suggested by the threshold effect analysis indicates the need for an even larger sample size to achieve significant results. Secondly, as this was a single-center retrospective study, the generalizability of our findings is limited, necessitating validation from multi-center global studies. Additionally, only patients in recovery from mild cases of COVID-19 were included, with moderate and severe cases unaccounted for. The short follow-up period also means that any long-term effects on abortion, live birth, and perinatal outcomes remain unknown. Lastly, the lack of data on vaccination status due to the historical nature of the control group is a notable limitation. However, according to the epidemic prevention policies at the time, the majority of the population had been vaccinated, and the current literature (41) suggests that vaccination status does not significantly impact clinical outcomes. In future, we will further trace long-term pregnancy outcomes as well as the health of the offspring, and further conduct multiple subgroup analyses of COVID-19-infected patients, considering variables such as the degree of fever, as well as comparing clinical outcomes between reinfected patients and those infected for the first time, so as to draw more comprehensive and reliable conclusions.

In conclusion, our findings suggest that a history of COVID-19 infection does not have a negative effect on clinical pregnancy rates; however, for women aged over 38 years, the clinical pregnancy rate in fresh transplant cycles was lower in both groups, but especially in the COVID-19 group. Specifically, the likelihood of clinical pregnancy declined by 53% for every additional year of age, indicating that COVID-19 further increases the burden of older age in women undergoing assisted reproductive therapy. In order to solve this problem and to offer reasoned and scientific suggestions or measures, we still need to complete more in-depth research.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/ Supplementary Material .

The studies involving humans were approved by Ethics Committee of the Second Hospital of Hebei Medical University (2022-R453). The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation was not required from the participants or the participants’ legal guardians/next of kin in accordance with the national legislation and institutional requirements.

MC: Data curation, Investigation, Methodology, Writing – original draft. YH: Data curation, Formal analysis, Investigation, Writing – review & editing. TF: Data curation, Software, Writing – review & editing. PL: Investigation, Writing – review & editing, Software. QS: Investigation, Writing – review & editing. YW: Supervision, Writing – review & editing. ZZ: Conceptualization, Writing – review & editing. WP: Writing – review & editing.