During the past year, we have been troubled by COVID-19, but now there is a glimmer of hope on the horizon. An effective vaccine has been developed. The UK and Russia have already started administering it, and in the United States and China, vaccination of the general population will begin soon.

The vaccine is surely good news, but not everyone feels hopeful and confident. There are also some people for whom safety is a concern, and here I would like to discuss and explain the safety of the vaccine.

To begin with, what exactly is a "safe vaccine"? Most people probably think that it is one that has no side effects. Consequently, when reports have been made of side effects among participants in clinical trials of the vaccine for COVID-19, there are no doubt people who do not want to be vaccinated right away even if it is approved. Among specialists who appear in the mass media, many are also very careful to add that while effectiveness of the vaccine has been established, its safety has not yet been confirmed.

The side effects that result from vaccination are called "side reactions," and today, vaccines that are widely used for measles, rubella, and polio and found to be effective and safe are not completely without side reactions either. Such side reactions include swelling of inoculated areas, swelling of lymph glands, and a slight fever. However, it is common knowledge that these side reactions alone, highly frequent, but mild, do not mean that the vaccine is not safe. For example, the measles and rubella combination vaccine widely used for infants causes fever between 37.5 and 38.5 degrees in 7 to 10 percent of infants receiving the vaccination and 20 percent show redness and swelling at the injection site.

But what about life-threatening reactions or critical side reactions that result in after-effects? From 2009 to 2013, for the measles/rubella vaccine, there were 25 reported cases of anaphylactic reaction, a serious allergic reaction, and 15 cases of encephalitis/encephalosis. During this period, total inoculation was 16,000,000, which means a frequency of 0.00025% (40 ÷ 16 million).

Such rare and serious side reactions appear for the first time when a large number of people are vaccinated. The currently approved COVID-19 vaccine has already been administered in clinical trials to about 40,000 people and there have been no reports of serious side reactions. However, if we assume that side reactions occur at a rate of 0.00025%, this means 40,000 x 0.0000025 = 0.1, an incidence of 0.1 person among 40,000 inoculated in a clinical trial, so the side reaction will appear in one person only after a tenfold number of people are vaccinated.

Unfortunately, in medicine today, it is not yet possible to completely eliminate these rare and serious side reactions. A vaccine that causes the serious side reactions in response to the combined measles/rubella vaccine is regarded as highly safe vaccine. Even the vaccines that are considered "safe" and currently used worldwide still have the possibility, albeit rare, of a serious side reaction.

Herd immunity and vaccination

I am writing about this here for a reason: if large numbers of people shun vaccination because they want to avoid rare side reactions, it will be difficult to achieve the necessary herd immunity throughout society to end the pandemic. I would like it to be clearly understood that the importance of vaccination against highly infectious measles, influenza or polio, and now against this new coronavirus is not only to protect the vaccinated from infection, but to also prevent the growing pandemic from spreading throughout society as a whole or to put an end to it completely.

When vaccinations were not yet available, thousands of children died every year from the measles. (In 1951, 9,000 deaths were reported, the highest number recorded!) But today when the vaccination rate has reached 98%, the number of deaths has declined to somewhere between 10 and 20. Although there are still cases of measles today, 98% of infants are now vaccinated, which has resulted in herd immunity that prevents a spread nation-wide.

In 1951 when there were 9000 deaths, the reported number of patients alone was 200,000, much higher than the current number of COVID-19 patients in Japan.

It is important to remember that by achieving herd immunity, it will be possible to also protect from infection those children who have not been vaccinated and do not have immunity.

Regarding the vaccine for COVID-19 as well, it is said that if herd immunity is achieved when about 60% of the population becomes immune through vaccination, those who are not vaccinated will also be protected. Vaccination for COVID-19 is effective in preventing the spread of the virus throughout the entire society including those who are not vaccinated. Some young people assume that the illness will be mild in their case because of their youth and believe that vaccination is unnecessary, so I would especially like to direct this fact to them. Regardless of whether your symptoms are mild or non-existent, there is still the possibility of infecting others.

I think that by the time vaccination is begun in Japan, we will have reliable data on serious side effects based on prior vaccination in the UK and the United States. However, as I have already explained, this does not mean that there will be zero serious side reactions. It is possible there will be serious side effects that are rare like those seen with the combined measles-rubella vaccine, and furthermore, even that level will be considered "safe." At present, clinical trials of 40,000 people have confirmed its effectiveness and safety and what we now know is that serious side effects are expected to be lower than 1/40000 (= 0.0025%).

However, as vaccination increases, we will see serious side reactions at a low frequency as seen in the measles-rubella vaccination, and as a physician, I fear that this will lead to people saying that "the vaccine is not safe," so to say an "infodemic" -i.e. the spread and overabundance of information, including questionable information, that causes social unrest.

Let's ensure that we have a correct understanding of the safety of the vaccine and take a level-headed approach to vaccination.