Although COVID-19 vaccine access has increased nationwide, vaccination rates have been slow-moving, with many studies showing significant vaccine hesitancy in the U.S. We conducted an online survey using Amazon Mechanical Turk (MTurk) to identify reasons for vaccine hesitancy among unvaccinated adults between June 30 and July 1, 2021. We found that 58% of unvaccinated respondents were worried about unknown long-term adverse effects. Of these, 41% believed that the COVID-19 vaccines can negatively impact reproductive health and or fertility, and 38% were unsure of the effects on fertility. Our study demonstrates that fear regarding COVID-19 vaccine adverse effects and belief that they can negatively impact fertility is a major cause of vaccine hesitancy in the United States. We identified that urban residents, married individuals, those born outside the U.S., those with health insurance, and people with higher education and income greater than $100,000 felt that the vaccine would affect fertility more than their counterparts did. Finally, we found that 48% of unvaccinated respondents cited ‘more information and research conducted on the COVID-19 vaccines’ as the action that would most encourage vaccine uptake.