To investigate adverse reactions and attitudes toward the vaccine during the first month after mRNA- 1273 vaccination in a larger sample including younger men and women in Japan, we distributed a 1-month post-vaccination questionnaire using a Google form to 8,566 people who received a second dose of mRNA-1273 at Okayama University. The response rate was about 40.2% (3,447 responses), the sex ratio was about the same, and 73.3 % (2,528 respondents) were students in their twenties or younger. Poisson regression with robust variance was performed to calculate the prevalence ratio of each symptom by different attributes. The most common adverse reactions after the second vaccine dose were local pain (80.4%), fever (85.1%), malaise (82.0%), headache (64.0%), and chills (57.4%). Approximately 99% of respondents reported that their adverse reactions resolved within 1 week. Over 80% of respondents were satisfied with their vaccination (87.2%), expressed interest in receiving the third vaccination (83.3%), and would recommend vaccination to their loved ones (80.2%). However, among them, 22.0% (757 respondents) would recommend and 28.4% (980 respondents) also stated that they would consider the type of vaccine in these decisions.