Study reports on COVID-19 vaccine-related adverse events in young Japanese population

The coronavirus disease 2019 (COVID-19) pandemic – caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – has lead to over 200 million confirmed cases worldwide. To date, over 4.2 million have lost their lives to the virus.

The rapid and extensive spread of the virus has made it necessary to implement sweeping public health interventions, such as social distancing, face mask use outside the home, school and business closures, and case isolation and contact tracing.

Due to the immense economic fallout of these measures, vaccination has long been held to be the only way out of this situation. Developed in record time, the Pfizer and Moderna vaccines, built on a messenger ribonucleic acid (mRNA) platform that encodes the viral spike antigen, were the first to receive emergency use authorization.

A new study from Japan, released as a medRxiv* preprint, describes the adverse effects associated with the vaccine. Such data is crucial in delineating the safety and efficacy of this intervention in various age groups.

Many developed countries have achieved vaccination of over half of their adult population, beginning with the elderly, more vulnerable subsets. In Japan, healthcare workers and people over 65 years were first vaccinated, followed by workplace vaccination at some corporations and universities. The COMIRNATY mRNA vaccine was used in the former category and the Moderna vaccine in the latter.

Study details

The researchers explored the adverse events reported after the administration of the first dose of the Comirnaty and Moderna vaccines. The questionnaire format was used to survey students, faculty and staff from an educational foundation called Kyushu Bunka Gakuen, after the first dose of the vaccine.

The foundation runs universities and junior colleges, high schools, as well as training facilities for cooking and dental hygienists. Local companies related to this foundation were included in the workplace vaccination cohort.

What were the findings?

Vaccination at the workplace was administered to approximately 4,000 people. All adverse events occurring within 30 minutes of administration of the vaccine were described. About 0.5% of vaccine recipients had adverse events, while one person with a history of anaphylaxis to the flu vaccine developed anaphylaxis to this vaccine too.

The analysis included approximately 1,900 participants, two-thirds of them being women. The median age was 22 years, but a quarter was below the age of 20.

Over 80% of the subjects reported a local vaccine adverse effect (VAE). The most common was pain, in 70%, followed by swelling and local heat in about 35-40%. Less frequent were redness and itching, in up to a quarter of cases. Most local events occurred within a day of injection and resolved spontaneously, though redness and itching at the injection site persisted in over a quarter, and just under a fifth, of the subjects, respectively.

Systemic VAE occurred in less than half, ranging from muscle pain/ tiredness in approximately one in three participants, respectively, fever/headache in a fifth of them, to joint pain or chills in 5%. Myalgia typically occurred on the first two days, resolving after three days at most.

Fatigue was also reported to occur on the same day, lasting for up to three days afterward. Most of the subjects did not seek medical help for these events, but about one in seven took acetaminophen.

Risk factors for VAE

The odds of local and systemic VAE were more than doubled in females relative to males. People with a history of allergy were also twice as likely to have local reactions, with systemic events being increased by 40%.

Minors (below 20 years of age) also had more local and systemic adverse events. They were 56% more likely to develop induration and itching at the site of injection.

Patients who had previously developed adverse reactions to other medications were three times more likely to complain of pain at the injection site, vs. 53% higher risk of systemic VAE.

What are the implications?

The findings of this study show that minors are more at risk for systemic VAE than older people, as are females. This finding is consistent with those of the Japanese Self-Defense Forces, and of the Ministry of Health, Labour, and Welfare Japan and the US Centers for Disease Control and Prevention (CDC).

The higher-than-expected incidence of VAE may be due to the larger proportion of young women. Myalgia and tiredness were very common, compared to headaches in other reports.

The researchers point out that available data on Japanese people who got two doses of the Comirnaty vaccine were more likely to develop VAE after the second dose. This is also likely with the Moderna vaccine. This prediction should be considered, especially since these vaccines are now being extended to young people.

The mitigating factors include the non-serious and self-limited nature of the VAE for the most part, coupled with 94% protection against symptomatic confirmed COVID-19 after two doses of the Moderna mRNA vaccine. This should be communicated clearly to prevent vaccine hesitancy, and acetaminophen provided to prevent severe events at this time, especially in those who experienced VAE after the first dose.

Further studies are being planned after the second dose is given. This will help understand what happens, and why, allowing better design and evaluation of mRNA vaccines.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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