Placebo

How should the safety of vaccines be tested? By performing randomized, double-blind, placebo-controlled clinical trials, during which the data on adverse effects experienced by the vaccine recipients is collected and compared to the control group.

Clinical trials are very expensive, they cost tens of millions of dollars. Drug development costs are estimated as hundreds of millions. But those amounts aren't an issue for the pharmaceutical companies. An FDA-approved vaccine very quickly becomes part of the vaccination schedules in most countries, and yields billions in profit on an annual basis. For example, sales of one of the recently licensed vaccines, Gardasil (against the HPV), account for more than 3 billion per year.

Naturally, pharmaceutical companies want to lower the odds of failing in the clinical trials. But do they have a legal possibility to do so?
It turns out they do, and it's implementation is quite simple. You just need to administer to the control group a fairly toxic substance instead of a placebo. Toxic in a way that it will lead to the same adverse effects as the vaccine being tested. One of the most toxic components of vaccines is aluminum (I elaborate on this in a dedicated section), which is used as an adjuvant in most "inactivated" vaccines. If an aluminum adjuvant, or aluminum with thimerosal, or simply another vaccine is used instead of a placebo, it is possible to increase the prevalence of adverse effects in the control group, so it will be comparable to the intervention group. This will allow the researchers to conclude that the new vaccine has no side effects, and it is completely safe. On this basis the FDA and the CDC will also be able to conclude that the vaccine is safe, and so will the countries around the world.
Is this legal? Absolutely.

Thing is, you don’t even have to worry over the choice of placebo. It is by no means mandatory to use a placebo in a randomized clinical vaccine study. The studies don’t even have to be randomized, or blind. You can simply give everyone the vaccine and observe what the adverse effects are. If the majority of the study participants remains alive, it means that the vaccine is absolutely safe.
Here are two very interesting articles:

No regulations govern placebo composition. The composition of placebos can influence trial outcomes and merits reporting. There is no requirement to disclose the contents of placebo used in clinical trials. Medical journals do not require this information either.
The authors analyzed 167 clinical studies published in four of the most prestigious medical journals. Most of the clinical studies did not disclose the composition of the placebo. Only studies for 8% of pills and 26% of injections reported what was used as the placebo. For example, in the study on medication for cancer-acquired anorexia, it was found that the medication positively affects the GIT. However, lactose was used as the placebo. Cancer patients who are undergoing chemo and radiation therapy usually acquire lactose intolerance, which resulted in the medication which did not contain lactose comparing favorably with the “placebo”.

In 1930, in the northern Germany town of Lübeck, two physicians mounted a campaign to vaccinate newborn babies against tuberculosis. They used a BCG vaccine produced locally from a Parisian strain. The vaccine had become available as early as 1921, but its use was still not widely accepted.
Soon after receiving the vaccine, infants were becoming seriously ill and many died. In 12 months, of all the infants vaccinated, 208 became ill with tuberculosis and 77 died. The two physicians were arrested and put on trial. The court eventually found the two physicians guilty of murder. The newspapers publicized the trial throughout Europe, and the news of the disastrous vaccine campaign spread throughout the continent.
This tragedy actually led to the first published discussion on medical research using human subjects, approximately two decades before the Nuremberg Trial and the Code of Nuremberg. Julius Moses, a Member of German Parliament, called for a public accounting of medical science. He charged that the vaccine campaign was in fact not a clinical practice proven safe and effective but rather an experimental trial. Furthermore, he decried the conduct of the campaign as research in humans without their knowledge or permission.
In 2008, the USA discontinued adopting the Declaration of Helsinki (instead, it uses Good Clinical Practice, which does not restrict pharmaceutical companies as much as the Declaration of Helsinki).
For intramuscular and subcutaneous vaccinations, injections of sterile normal saline may serve as placebos, but researchers frequently choose other comparative agents. A review of the most recently published trials involving vaccines for children found a variety of comparators that took the place of placebos in children.
For example, a study of pneumococcal conjugate vaccine with nine serotypes (PCV-9) used the DTP-Hib vaccine as its comparator.
In another study, a novel oral cholera vaccine was compared to E. coli vaccine.
In another study on a vaccine for pneumococcus (PCV23), vaccines for hepatitis A and B were used as comparators.
In a fourth study, the study vaccine was compared to aluminum hydroxide combined with thimerosal.

Unlike regular medications clinical trials, in which the composition of the placebo is often undisclosed, many vaccine manufacturers do not usually conceal the placebo they used. In order to find out the placebo composition you just need to read the vaccine insert. Here are only a few examples.

Daptacel, a vaccine for diphtheria, tetanus and whooping cough (DTaP, Sanofi Pasteur). Three vaccines were used as the placebo: DTP, DT and an experimental vaccine for whooping cough.
This is not a mistake. An experimental vaccine was used as a placebo. Let that sink in.

Infanrix, another vaccine for diphtheria, tetanus and whooping cough (DTaP, GlaxoSmithKline). The vaccine Pediarix was used as the placebo. In addition, both groups received those vaccines alongside with vaccinations for hepatitis B, pneumococcus, chickenpox, poliovirus, haemophilus influenzae, measles, mumps and rubella.

Pediarix, a vaccine for diphtheria, tetanus, whooping cough, hepatitis B and polio (DTaP-HepB-IPV, GlaxoSmithKline). This vaccine was tested together with a vaccine for haemophilus influenzae. The control group received the vaccine Infanrix, as well as a vaccine for polio and haemophilus influenzae.

In other words, Pediarix was used as the placebo in the Infanrix trials, and Infanrix was used as the placebo in the Pediarix trials. All of this was seasoned with a blend of several other vaccines, in order to completely eradicate the possibility of distinguishing any adverse effects resulting from the vaccines being tested.

The first vaccines for diphtheria, tetanus and whooping cough appeared long before anyone bothered with clinical trials or the use of placebos. Thus, here it can be argued that the use of a placebo to test them, i.e. not to vaccinate a part of the participating children, is unethical. But even the clinical trials of vaccines against diseases that weren't "vaccine-preventable" at the time, use other vaccines as the placebo:

Havrix, a vaccine for hepatitis A (GlaxoSmithKline). The clinical study included three groups. The first one received Havrix. The second one received Havrix + MMR (measles/mumps/rubella vaccine). The third received MMR + chickenpox, as then Havrix 42 days after.

Prevnar, pneumococcal vaccine (PCV7, Wyeth). The placebo was an experimental (!) meningococcal C vaccine.
The next version of this vaccine Prevnar-13 (Pfizer) already used Prevnar as a placebo.

Cervarix, human papillomavirus vaccine (GlaxoSmithKline). The placebo was a hepatitis A vaccine, as well as aluminum hydroxide.

Engerix-B, a vaccine for hepatitis B (GlaxoSmithKline). There was no control group.

Recombivax, a vaccine for hepatitis B (Merck). There was no control group.

In order to license a new vaccine, it is sufficient to show the FDA that it is no more dangerous than some another vaccine (which may be an experimental one), or than aluminum hydroxide, or than any other substance the pharmaceutical company sees fit, with no obligation to publicly disclose.
In vaccines clinical studies, a true, inert placebo is virtually never used.
So next time someone claims that vaccines are completely safe, ask them: "compared to what?"
Vaccines are only completely safe when compared to other vaccines, or to extremely toxic substances.