Pre-clinical trials were conducted in mice which showed that two doses of the vaccine protected the mice from infection with the SARS-CoV-2 virus with no virus detected in the lower respiratory tract and nose following a challenge with the SARS-CoV-2 virus. These results indicated the vaccine may reduce both infection and transmission in humans.

The US phase 1 trial included participants aged 18-55 years old who received two injections of Spikevax 28 days apart at doses of 25 μg, 100 μg or 250 μg. The vaccine elicited a strong immune response without any severe adverse events and the trial supported the selection of the 100 μg  dose for advancement to Phase 2/3 trials.

Safety profile

During clinical trials, symptoms following immunisation were generally mild to moderate and short-term. Local and systemic reactions were more common after the second dose and in the younger age group (18-64 years).

Local reactions included pain (86-90%) and redness (3-9%) at the injection site and axillary (armpit) swelling/tenderness (11-16%). The average onset of local reactions was 1 day after either dose, with an average duration of 2-3 days.

Systemic reactions (including fatigue, headache,  muscle ache, joint pain, chills, nausea and vomiting) were more common after the second dose (82%) compared to the first dose (57%).  Fever was more common following the second dose (17%). The average onset of local reactions was 1-2 days after either dose, with an average duration of 2 days. The incidence of serious adverse events was very low (1%) and similar in both the vaccine and placebo groups.

Vaccine efficacy

phase 3 trial enrolling 30,420 volunteers aged ≥18 years, assigned participants to receive either the vaccine or placebo as two intramuscular injections 28 days apart. Spikevax showed overall efficacy of 94.1% for preventing COVID-19 illness, and 100% efficacy for preventing severe disease. Vaccine efficacy was consistent across subgroups stratified by age group (18 to < 65 years of age, ≥ 65 years, presence of risk for severe COVID-19 disease, sex, race and ethnic groups).

Paediatric clinical trials

3732 adolescents (12-15 years) participated in a phase 3 trial in the US investigating the safety and efficacy of Spikevax (Moderna) in children. Results from this trial have demonstrated an efficacy against confirmed symptomatic COVID-19 disease of 93.3% (commencing 14 days after dose 2 of vaccine. 1 case in vaccine group, 7 cases in placebo group). There were no cases of severe COVID-19 or deaths in the study cohort.

Up to 4000 children (6-11 years) were recruited to the KidCOVE clinical trial across Canada and the United States. The immunogenicity, safety and efficacy of 2 doses of 50µ of Spikevax (Moderna) was assessed. Immunogenicity was comparable to that seen in individuals aged 18-25, with a good protection seen against COVID-19 disease.

2024 children aged 6 to 23 months and 3452 children aged 2-5 years participated in the phase 2-3 P205 clinical trial assessing the immunogenicity, efficacy and safety of a two-dose primary course of 25µ of Spikevax (Moderna). The immune response of the infants and children was comparable to that seen in adults aged 18-25 years in earlier trials. Results (>14 days after dose 2) demonstrated that in those without history of prior COVID-19 infection, vaccination was 50.6% effective in preventing disease in participants aged 6-23 months of age, and 36.8% in those aged 2-5 years. In those with a history of previous COVID-19 infection, vaccine efficacy against symptomatic COVID-19 disease was 52.1% for the 6-23 month group, and 34.5% in the 2-5 years group. There were no cases of severe COVID-19 disease reported in the study.

For further information, refer to MVEC: COVID-19 vaccination in children.

Post-licensure surveillance


A true vaccine allergy (anaphylaxis) is a rare side effect occurring for all vaccines at a rate of approximately 1 case per million vaccine doses administered. Post-licensure surveillance of Spikevax in the US has shown that it has a slightly higher rate of anaphylaxis with approximately 2.5 cases per million. Most cases (89%) occurred within 30 minutes of vaccination and 26% had a history of prior anaphylaxis.


A small number of cases of myocarditis and pericarditis have been reported in individuals vaccinated with COVID-19 mRNA vaccines (eg. Comirnaty and Spikevax). Reports have predominantly involved adolescents and young adults, more commonly males, after the second dose of vaccine. Symptom onset has typically been seen within 4 days of vaccination.


Authors: MVEC Education Team

Reviewed by: Rachael McGuire, (MVEC Education Nurse Coordinator) and Francesca Machingaifa (MVEC Education Nurse Coordinator)

Date: September 15, 2022

Materials in this section are updated as new information and vaccines become available. The Melbourne Vaccine Education Centre (MVEC) staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family’s personal health. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult a healthcare professional.