Vaccines and immunization: Myths and misconceptions

19 October 2020 | Q&A

This list of common misconceptions was originally written by the Centers for Disease Control and Prevention in the United States, primarily for use by practitioners giving vaccinations to children in their practices. An edited version is reproduced here as useful information for health-care workers giving vaccination as well as concerned parents. This Q&A addresses the common misconceptions about vaccination that are often cited by concerned parents as reasons to question the wisdom of having their children vaccinated. WHO gratefully acknowledges the permission of CDC Atlanta to present an edited version of "Six common misconceptions about immunization".

Ideas like this are very common in anti-vaccine literature, the intent apparently being to suggest that vaccines are not needed. Improved socioeconomic conditions have undoubtedly had an indirect impact on disease. Better nutrition, not to mention the development of antibiotics and other treatments, have increased survival rates among the sick; less crowded living conditions have reduced disease transmission; and lower birth rates have decreased the number of susceptible household contacts. However, looking at the actual incidence of disease over the years can leave little doubt of the significant direct impact vaccines have had, even in modern times.

For example, since sanitation is not better now than it was in 1990, it is hard to attribute the virtual disappearance of diseases such as Haemophilus influenzae type b (Hib) in children in recent years in countries with routine Hib vaccination (from an estimated 20 000 cases a year to 1419 cases in 1993, and dropping, in the United States of America) to anything other than the vaccine.

There are many examples, including smallpox, which was eradicated worldwide in 1979 following a collaborative global vaccination programme led by the World Health Organization. Smallpox killed hundreds of millions of people and was one of the most feared diseases for over 3000 years. Today is completely gone thanks to immunization efforts.

Another example is measles. There have been periodic peaks and valleys throughout the years, but the real, permanent drop in measles incidence coincided with the licensure and wide use of measles vaccine beginning in 1963. Other vaccine-preventable diseases show a roughly similar pattern in incidence, with all except hepatitis B showing a significant drop in cases corresponding with the advent of vaccine use.

The incidence of hepatitis B has not dropped as much because infants vaccinated in routine programs will not be at high risk of disease until they are at least teenagers. Therefore, a 15-year lag can be expected between the start of routine infant vaccination and a significant drop in disease incidence. The Haemophilus influenzae type b (Hib) vaccine had a similar period of delay. Hib disease was prevalent until the early- to mid-1990s, when conjugate vaccines that can be used for infants were finally developed. The polysaccharide vaccine previously available could not be used for infants, in whom most cases of the disease were occurring.

We can look at the experiences of several developed countries after they allowed their immunization levels to drop. Three countries – the United Kingdom of Great Britain and Northern Ireland, Sweden and Japan – cut back the use of pertussis (whooping cough) vaccine because of fear about the vaccine. The effect was dramatic and immediate. In the United Kingdom of Great Britain and Northern Ireland, a drop in pertussis vaccination in 1974 was followed by an epidemic of more than 100 000 cases of pertussis and 36 deaths by 1978. In Japan at around the same time, a drop in vaccination rates from 70% to 20–40% led to a jump in pertussis from 393 cases and no deaths in 1974 to 13 000 cases and 41 deaths in 1979. In Sweden, the annual incidence rate of pertussis per 100 000 children of 0–6 years of age increased from 700 cases in 1981 to 3200 in 1985.

There were also major epidemics of diphtheria that occurred in the former Soviet Union in the 1990s, where low primary immunization rates for children and the lack of booster vaccinations for adults resulted in an increase from 839 cases in 1989 to nearly 50 000 cases and 1700 deaths in 1994. There were at least 20 imported cases in Europe and two cases in U.S. citizens who had worked in the former Soviet Union.
It seems clear from these experiences that not only would diseases not be disappearing without vaccines, but if we were to stop vaccinating, they would come back.

This is another argument frequently found in anti-vaccine literature, the implication being that this proves that vaccines are not effective. In fact it is true that in an outbreak those who have been vaccinated often outnumber those who have not — even with vaccines such as measles, which we know to be about 98% effective when used as recommended.

This apparent paradox is explained by two factors. First, no vaccine is 100% effective. To make vaccines safer than the disease, the bacteria or virus is killed or weakened (attenuated). For reasons related to the individual, not all vaccinated persons develop immunity. Most routine childhood vaccines are effective for 85% to 95% of recipients. Second, in most high-income countries, the people who have been vaccinated vastly outnumber those who have not.

How these two factors work together to result in outbreaks in which the majority of cases have been vaccinated can be more easily understood by looking at a hypothetical example:

Consider a high school of 1,000 students where none has ever had measles. All but 5 of the students have had 2 doses of measles vaccine, and so are fully immunized. The entire student body is exposed to measles, and every susceptible student becomes infected. The 5 unvaccinated students will be infected, of course. But of the 995 who have been vaccinated, we would expect several not to respond to the vaccine. The efficacy rate for 2 doses of measles vaccine can be as high as >99%. In this class, 7 students do not respond, and they, too, become infected. Therefore 7 of 12, or about 58%, of the cases occur in students who have been fully vaccinated.

As you can see, this doesn't prove the vaccine didn't work — only that most of the children in the class had been vaccinated, so those who were vaccinated and did not respond outnumbered those who had not been vaccinated. Looking at it another way, 100% of the children who had not been vaccinated got measles, compared with less than 1% of those who had been vaccinated. Measles vaccine protected most of the class; if nobody in the class had been vaccinated, there would probably have been 1,000 cases of measles.

This misconception often receives considerable publicity. First, the concept of a "hot lot" of vaccine as it is used in this context is wrong. It is based on the presumption that the more reports of adverse events a vaccine lot is associated with, the more dangerous the vaccine is in that lot. It also suggests that by consulting a list of the number of reports per lot, a parent can identify vaccine lots to avoid.

This is misleading for two reasons. First, an adverse report following vaccination does not mean that the vaccine caused the event. Statistically, a certain number of serious illnesses, even deaths, can be expected to occur by chance alone among children recently vaccinated. Although vaccines are known to cause minor, temporary side effects such as soreness or fever, there is little, if any, evidence linking vaccination with permanent health problems or death.

Second, vaccine lots are not the same. The sizes of vaccine lots might vary from several hundred thousand doses to several million, and some are in distribution much longer than others. Naturally, a larger lot or one that is in distribution for a longer period will be associated with more adverse events, simply by chance. Also, more coincidental deaths are associated with vaccines given in infancy than later in childhood, since the background death rates for children are highest during the first year of life. Knowing that lot A has been associated with x number of adverse events while lot B has been associated with y number would not necessarily say anything about the relative safety of the two lots.

Reviewing published lists of "hot lots" will not help parents identify the best or worst vaccines for their children. If the number and type of adverse event reports for a particular vaccine lot suggested that it was associated with more serious adverse events or deaths than are expected by chance, most countries have a system to recall those vaccines. All vaccines purchased through the UNICEF vaccine procurement system meet World Health Organization standards for safety and quality of production.

Vaccines are very safe, despite implications to the contrary in many anti-vaccine publications. Most vaccine adverse events are minor and temporary, such as a sore arm or mild fever. These can often be controlled by taking paracetamol after vaccination. More serious adverse events occur rarely (on the order of one per thousands to one per millions of doses), and some are so rare that risk cannot be accurately assessed. So few deaths can plausibly be attributed to vaccines that it is hard to assess the risk statistically. Each death reported to ministries of health is thoroughly examined to assess whether it is really related to administration of vaccine, and if so, what exactly is the cause. When, after careful investigation, an event is felt to be a genuine vaccine-related event, it is most frequently found to be a programmatic error, not related to vaccine manufacture.

One myth that won't seem to go away is that the DTP vaccine causes sudden infant death syndrome (SIDS). This belief came about because a moderate proportion of children who die of SIDS have recently been vaccinated with DTP; on the surface, this seems to point toward a causal connection. This logic is faulty, however; you might as well say that eating bread causes car crashes, since most drivers who crash their cars could probably be shown to have eaten bread within the past 24 hours.

If you consider that most SIDS deaths occur during the age range when three shots of DTP are given, you would expect DTP shots to precede a fair number of SIDS deaths simply by chance. In fact, when a number of well-controlled studies were conducted during the 1980s, the investigators found, nearly unanimously, that the number of SIDS deaths temporally associated with DTP vaccination was within the range expected to occur by chance. In other words, the SIDS deaths would have occurred even if no vaccinations had been given.

In fact, in several of the studies, children who had recently received a DTP shot were less likely to get SIDS. The Institute of Medicine reported, "all controlled studies that have compared immunized versus non-immunized children have found either no association . . . or a decreased risk . . . of SIDS among immunized children," and concluded, "the evidence does not indicate a causal relation between [DTP] vaccine and SIDS."

Looking at risk alone is not enough; you must always look at both risks and benefits. Even one serious adverse effect in a million doses of vaccine cannot be justified if there is no benefit from the vaccination. However, if there were no vaccines, there would be many more cases of disease, and along with them, more serious side effects and more deaths. For example, according to an analysis of the benefit and risk of DTP immunization, if there was no immunization program in the United States, pertussis cases could increase 71-fold and deaths due to pertussis could increase 4-fold. Comparing the risk from disease with the risk from the vaccines can give us an idea of the benefits we get from vaccinating our children.

The fact is that a child is far more likely to be seriously injured by one of these diseases than by any vaccine. While any serious injury or death caused by vaccines is too many, it is also clear that the benefits of vaccination greatly outweigh the slight risk, and that many, many more injuries and deaths would occur without vaccinations. In fact, to have a medical intervention as effective as vaccination in preventing disease and not use it would be unconscionable.

It is true that vaccination has enabled us to reduce most vaccine-preventable diseases to very low levels in many countries. However, some of them are still quite prevalent – even epidemic – in other parts of the world. Travellers can unknowingly bring these diseases into any country, and if the community were not protected by vaccinations, these diseases could quickly spread throughout the population, causing epidemics there.

At the same time, the relatively few cases that a country may currently have could very quickly become tens or hundreds of thousands of cases without the protection given by vaccines. We should therefore still be vaccinated. There are two main reasons. The first is to protect ourselves. Even if we think our chances of getting any of these diseases are small, the diseases still exist and can still infect anyone who is not protected.

The second is to protect those around us. There is a small number of people who cannot be vaccinated (because of severe allergies to vaccine components, for example), and a small percentage of people don't respond to vaccines. These people are susceptible to disease, and their only hope of protection is that people around them are immune and cannot pass disease on to them. A successful vaccination program, like a successful society, depends on the cooperation of every individual to ensure the good of all. We would think it irresponsible of a driver to ignore all traffic regulations on the presumption that other drivers will watch out for them. In the same way, we shouldn't rely on people around us to stop the spread of disease; we must all do what we can.

Children are exposed to many foreign antigens every day. Eating food introduces new bacteria into the body, and numerous bacteria live in the mouth and nose, exposing the immune system to still more antigens. An upper respiratory viral infection exposes a child to 4–10 antigens, and a case of strep throat to 25–50. According to "Adverse events Associated with childhood vaccines", a 1994 report from the Institute of Medicine in the United States, "In the face of these normal events, it seems unlikely that the number of separate antigens contained in childhood vaccines . . . would represent an appreciable added burden on the immune system that would be immuno-suppressive."

Indeed, available scientific data show that simultaneous vaccination with multiple vaccines has no adverse effect on the normal childhood immune system. A number of studies and reviews have been conducted to examine the effects of giving various combinations of vaccines simultaneously. These studies have shown that the recommended vaccines are as effective in combination as they are individually, and that such combinations carry no greater risk for adverse side effects.

Research is under way to find ways to combine more antigens in a single vaccine injection, as with the measles, mumps rubella (MMR) vaccine. This method provides all the advantages of the individual vaccines but will require fewer injections. There are two practical factors in favour of merging vaccines and giving a child several vaccinations during the same visit. First, we want to immunize children as early as possible to give them protection during the vulnerable early months of their lives. This generally means giving inactivated vaccines beginning at 2 months and live vaccines at 12 months. The various vaccine doses thus tend to fall due at the same time. Second, combining several vaccinations will mean fewer clinic visits for vaccinations, which saves parents both time and money and may be less traumatic for the child. In countries where there is a likelihood of reduced contact with the health care system, there is an added advantage of ensuring that there are no missed opportunities to complete the recommended vaccinations for a child.

Related Q&A

Immunization and vaccines: vaccine safety