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.
The most reliable source of COVID-19 vaccine information in Japan is, I believe, the content posted on the Ministry of Health, Labour and Welfare (MHLW) website.
However, materials such as the health status surveys are quite difficult to follow, so few people are likely to read them.
The MHLW’s Q&A on COVID-19 vaccines is written very clearly, so I will quote from it here.
Vaccine basics
First, two COVID-19 vaccines are currently being administered in Japan: the Pfizer vaccine and the Moderna vaccine.
Both are mRNA vaccines that work through 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 practice most people I know said it hurt about the same as a subcutaneous injection such as a flu shot, or even less.
There are some other minor differences, but fundamentally both offer comparable efficacy, and current data indicate that neither is superior to the other.
Main side effects of the vaccines
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.
Injection-site pain more often appears the day after vaccination than immediately afterward, and side effects can be somewhat delayed, so take good care of yourself on the day after your shot.
In rare cases, 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 a health problem arises from side effects after vaccination, you may be able to use the Relief System for Injury to Health with Vaccination; contact your local municipality for details.
A common side effect of the Moderna vaccine is what is popularly 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 a day to a few days.
Side effects are also said to be more likely after the second dose than after the first.
Health survey reported by the MHLW (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 greatly 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 sensation of heat 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 the proportion affected roughly halves on day 3 and again on day 4.
Regarding systemic reactions, fever above 37.5°C occurred in under 10% of people after the first dose, compared with about 40% after the second.
Fever was more common in younger people (about 50% among those in their 20s), and roughly 5 percentage points more common in women than in men.
Fever peaks on day 2, but by day 3 the proportion affected drops to about one-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 symptoms, 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, as were the symptoms themselves.
As for “Moderna arm,” which is characteristic of the Moderna vaccine, it occurs in about 5% of people after the first dose.
Five percent may sound rare, but it works out to one in twenty people, so most 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.
- Almost everyone experiences pain at the injection site
- Local symptoms such as pain, redness, and swelling at the injection site occur at roughly the same rate after the first and second doses
- Systemic reactions occur more often after the second dose than the first
- Fever and fatigue can occur in many people
- Side effects are more common in younger people, and more common in women than in men
- Side effects peak the day after vaccination, and for most people symptoms subside within one to three days
A great deal of media coverage is framed in ways that stoke anxiety.
By looking at the actual data with your own eyes, you can get a clearer sense of what the concerns about COVID-19 vaccines really amount to.
I hope this article is helpful to you.