This article was automatically translated from Japanese using AI. The Japanese version is the authoritative version.
With misinformation circulating about the side effects of COVID-19 vaccines and media coverage that often stokes anxiety, it can be hard to know which information to trust.
In Japan, the most reliable source of information on COVID-19 vaccines is, I believe, the content posted on the Ministry of Health, Labour and Welfare website.
However, materials such as the health status surveys are quite difficult to follow, so few people are likely to read them.
The Ministry’s Q&A on COVID-19 vaccines is written very clearly, so I will cite it here.
Basic facts about the vaccines
First, two COVID-19 vaccines are currently being administered in Japan: the Pfizer vaccine and the Moderna vaccine.
Both are mRNA vaccines that work by essentially the same mechanism, and their efficacy in preventing symptomatic COVID-19 is nearly identical—about 95% for Pfizer and about 94% for Moderna.
Both are given as intramuscular injections.
Intramuscular injection may look painful because the needle goes in fairly deep, but in fact most people around me said it hurt about as much as a subcutaneous injection such as a flu shot, or even less.
There are a few other minor differences, but both vaccines are basically equally effective, and current data show no clear advantage of one over the other.
Common side effects
Side effects that may occur with both the Pfizer and Moderna vaccines include fever, headache, joint and muscle pain, pain at the injection site, fatigue, and chills.
Pain at the injection site often appears the day after vaccination rather than immediately, and side effects can be delayed, so pay close attention to how you feel on the day after your shot.
Rarely, serious reactions such as shock or anaphylaxis (a severe allergic reaction) may occur.
Very rarely, mild myocarditis and pericarditis have been reported after vaccination.
If you experience health damage from side effects after vaccination, you can apply to the Relief System for Injury to Health with Vaccination, so contact your local government office.
A side effect commonly seen with the Moderna vaccine is what is known as “Moderna arm.”
This refers to pain and swelling at the injection site appearing about a week after vaccination.
In general, side effects subside within one to several days.
Side effects are also said to be more likely after the second dose than the first.
Health survey reported by the Ministry of Health, Labour and Welfare (as of July 7)
The link to the health survey is here
Pfizer vaccine
Please see the link for the detailed data.
Here I will give a rough summary of what the data show.
As for local reactions at the injection site, the proportion of people with redness or swelling did not differ much between the first and second doses, at roughly 10–12%.
However, about 90% of people reported pain at the injection site after both the first and second doses, so nearly everyone experiences some pain.
The proportion reporting a feeling of warmth at the injection site was about 10% after the first dose and about 16% after the second.
These symptoms peak on the day after vaccination and then roughly halve on day 3 and again on day 4.
As for systemic reactions, fever above 37.5°C occurred in under 10% of people after the first dose, compared with about 40% after the second dose.
Fever is more common in younger people (about 50% among those in their 20s), and about 5 percentage points more common in women than in men.
Fever peaks on day 2, but by day 3 the proportion drops to about a quarter or less, and by day 4 it has resolved in nearly everyone—so it clears up quickly for most people.
General fatigue occurred in about 25% of people after the first dose and about 70% after the second, while headache occurred in about 20% after the first dose and about 55% after the second. As with fever, the trend is that younger people are more likely to experience these effects, and women more so than men.
The Moderna vaccine
The rates of side effects for the Moderna vaccine were largely the same as for Pfizer, and the symptoms were similar as well.
As for “Moderna arm,” which is characteristic of the Moderna vaccine, it occurred in about 5% of people after the first dose.
Five percent may sound like a rare symptom, but it amounts to one in twenty people, so many of us probably know one or two people who have experienced it.
Summary
I have covered a lot of ground on the rates of side effects, so let me summarize here.
- Pain at the injection site occurs in almost everyone
- Local symptoms such as pain, redness, and swelling at the injection site occur at almost the same rate after the first and second doses
- Systemic reactions occur at a higher rate after the second dose than after the first
- Fever and fatigue may occur in a large proportion of people
- Side effects are more likely in younger people, and more likely in women than in men
- Side effects peak the day after vaccination, and in most people the symptoms subside within one to three days
A great deal of media coverage is framed in a way that fuels anxiety.
By looking at the actual data for yourself, you may get a clearer sense of just how much there really is to worry about with the COVID-19 vaccine.
I hope this article proves helpful to you.