Intense itching on both arms; Both injection sites were slightly enlarged; Both injection sites were red; Both arms had become swollen; Aches and pains; Flu-like symptoms; Lost two pounds due to the nausea; Nausea; Pain in both arms; Chills; Received a half dose of the Moderna COVID19 Vaccine in both arms 30 minutes apart; A spontaneous report  was received from a consumer, a 75-year-old female patient, who was administered Moderna's COVID-19 vaccine (mRNA-1273) and experienced received a half dose of the Moderna COVID19 Vaccine in both arms 30 minutes apart, flu-like symptoms, nausea, chills, aches and pains, pain in both arms, lost two pounds due to the nausea, both arms had become swollen, intense itching on both arms, both injection sites were red, and both injection sites were slightly enlarged.  The patient's medical history included low weight and family history of vaccine problems. Concomitant product use was not provided by the reporter.   On 21 Jan 2021, a day prior to the onset of the events, the patient received their first of two planned doses of mRNA-1273 intramuscularly in both arms for prophylaxis of COVID-19 infection.   The patient reported that she received a half dose of the vaccine in both arms 30 minutes apart in her immunologist's office. The patient noted that she had received pediatric doses in other instances due to her low weight.   On 22 Jan 2021, the patient experienced flu-like symptoms including nausea, chills, aches and pains, and pain in both arms on.  The patient reported that she lost two pounds due to the nausea.   On 23-JAN-2021, the patient was feeling substantially better.   On 25 Jan 2021, the patient noticed that both her arms had become swollen.   On 30 Jan 2021, the patient began experiencing intense itchiness on both arms and both injection sites were red and slightly enlarged. Treatment information was not provided.  The Action taken with the second dose of mRNA-1273 in response to the event was not reported.   The outcome of the event, received a half dose of the Moderna COVID19 Vaccine in both arms 30 minutes apart, was considered resolved on 21 Jan 2021. The outcome of the events, flu-like symptoms, nausea, chills, aches and pains, and pain in both arms, were considered resolving. The outcome of the events, lost two pounds due to nausea, both arms had become swollen, intense itchiness on both arms, both injection sites were red, and both injection sites were slightly enlarged, were unknown.; Reporter's Comments: This spontaneous report  concerns a 75-year-old female patient, with family history of vaccine problems (unspecified)who w experienced received a half dose of the Moderna COVID19 Vaccine in both arms 30 minutes apart, flu-like symptoms, nausea, chills, aches and pains, pain in both arms, lost two pounds due to the nausea, both arms had become swollen, intense itching on both arms, both injection sites were red, and both injection sites were slightly enlarged. The events of flu-like symptoms nausea, chills, aches and pains, and pain in both arms, lost two pounds due to the nausea occurred 1 day after the administration of mRNA-1273 vaccine (Lot #:  unknown, expiration date: unknown) and arms had become swollen occurred 3 days after vaccine administration.  The events of itchiness on both arms and both injection sites were red and slightly enlarged occurred 4 days after mRNA-1273 vaccine. Treatment provided was not reported and lost two pounds due to nausea, swollen and intense itchiness on arms, injection sites were red and slightly enlarged, were unknown. Flu-like symptoms, nausea, chills, aches and pains, and pain in both arms, were considered resolving. Based on the information provided which shows a temporal association a causal association, between the reported events and the administration of mRNA-1273 vaccine cannot be excluded. The events of received a half dose of the Moderna COVID19 Vaccine in both arms 30 minutes apart (Product administration error) is not applicable.