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General Practice 2023

COVID-19 vaccines: effectiveness, doses and side effects

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COVID-19 vaccines: effectiveness, doses and side effects
COVID-19 vaccines: effectiveness, doses and side effects

Several vaccines against COVID-19 are being studied and developed around the world to try to combat the pandemic caused by the new coronavirus.

To date, the main vaccines approved for emergency use by the WHO [1] are:

  • Pfizer and BioNTech (Comirnaty): the vaccine was 95% effective against infection and 100% against severe cases of the disease;
  • Modern (Spikevax): the vaccine was 94.1% effective against infection and 95% against severe cases of the disease;
  • AstraZeneca (Vaxzevria): the vaccine demonstrated efficacy of 63.09% against infection;
  • Sinovac (Coronavac): demonstrated an efficacy rate of 51% for mild cases and 100% for moderate and severe infections;
  • Serum Institute (Covishield): no official WHO report on efficacy;
  • Johnson & Johnson/ Janssen (JNJ-78436735): had an effectiveness rate of 66.9%, and this rate varied according to the country where it was applied. It is also 100% effective against severe cases of COVID-19 and hospitalization;
  • Sinopharm (Vero Cell): demonstrated 79% efficacy against the onset of infection and 79% against hospitalization;
  • Baharat Biotech (Covaxin): showed 78% efficacy against COVID-19 infection and 93% against severe infection;
  • Serum Institute (Covovax): no official WHO report on efficacy;
  • Novavax (Nuvaxovid): no official WHO report on efficacy.

Vaccines used in Brazil and Portugal

The vaccines against COVID-19 that are in use in Brazil [2] are the vaccine from Pfizer and BioNTech; Coronavac; the Johnson & Johnson vaccine; and the AstraZeneca vaccine.

In Portugal [3], the approved vaccines are those of Pfizer and BioNTech; that of Modern; the Johnson & Johnson vaccine; the AstraZeneca vaccine; and the Novavax vaccine.

How COVID-19 vaccines work

Vaccines against COVID-19 have been developed based on 4 types of technology:

  • Genetic technology (mRNA or DNA) (Pfizer and Modern): is a technology that makes he althy cells in the body produce the same protein that the coronavirus uses to enter in the cells. In doing so, the immune system is forced to produce antibodies that, during an infection, can neutralize the true coronavirus protein and prevent the infection from developing;
  • Use of viral/modified adenovirus vectors (Astrazeneca, Sputnik V and Janssen): consists of using adenoviruses, which are harmless to the human body, and genetically modifying them to that act similarly to the coronavirus, but without risk to he alth. This causes the immune system to train and produce antibodies capable of eliminating the virus if infection occurs;
  • Use of proteins or protein fragments (Novavax): use a part, or the complete protein, of the virus that binds to cells, to "train" the immune system to knowing which proteins to recognize and attack during an actual infection;
  • Use of inactivated coronavirus (Coronavac): an inactivated form of the new coronavirus is used that does not cause infection or he alth problems, but allows the body to produce the antibodies needed to fight the virus.

All these ways of working are theoretically effective and already work in the production of vaccines for other diseases. Check out the most common questions about the COVID-19 vaccine.

How many doses of vaccine are needed?

The primary vaccination schedule includes the following doses, according to the vaccine:

  • Coronavac: 2 doses, 2 to 4 weeks apart;
  • Pfizer and BioNTech: 2 doses, 8 weeks apart;
  • Modern: 2 doses, 28 days apart;
  • Astrazeneca: 2 doses, 8 weeks apart;
  • Johnson & Johnson/Janssen: 1 single dose.

For vaccines that require two applications, the WHO recommends that both doses be from the same laboratory. For now, there is no recognized benefit in the use of different vaccine doses.

After the application of the doses of the vaccination schedule, the WHO and the Ministries of He alth of different countries still recommend booster doses, which guarantee the effectiveness of the vaccines for a longer time.

When to take the booster dose?

The Ministry of He alth in Brazil has authorized the booster dose of the COVID-19 vaccine for all persons over 18 years of age. This dose should be taken after a minimum interval of 4 months from the initial doses [4] In addition, a second booster dose (4th dose) is indicated for people over 40 and he althcare workers, and it is recommended that this dose be taken 4 months after the 1st booster dose (3rd dose).

In Portugal, the booster dose is approved for people over 50 years of age, living in nursing homes or similar institutions, he alth professionals, people over 12 years of age and with increased risk pathologies or people with trisomy 21 and age over 16 years.

Learn more about when to get your third dose of COVID-19 vaccine.

Is the vaccine effective against the new variants?

According to the WHO [5], vaccines against COVID-19 should have an effect against emerging variants of the virus, as they stimulate a complex immune response of the whole organism, which will be "watchful" for particles of the new coronavirus, even if there are some changes in its structure.

Still, even if you become infected with a new variant, the chances of developing a serious, life-threatening infection are much lower for someone who is fully immunized, that is, more than 2 weeks later. the 2nd dose of the vaccine.

It is expected that over time, and as new variants emerge, the composition of vaccines will be gradually updated to provide greater protection.

How long does it take to take effect?

The protective effect of the vaccine against COVID-19 may take a few weeks, as the body needs time to be able to produce the antibodies that will guarantee immunity against the infection.

Also, in the case of vaccines that require 2 doses, protection is only guaranteed 2 to 3 weeks after the 2nd dose.


Can children and adolescents be vaccinated?

The recommendations of the Center for Disease Treatment and Prevention (CDC) is that all children and adolescents over 5 years of age be vaccinated against COVID, and the administration of the Pfizer vaccine up to the age of 18 is recommended[6].

Currently, in Brazil, ANVISA and the Ministry of He alth have authorized the use of the Pfizer vaccine in children over 12 years old, as well as the pediatric version in children between 5 and 11 years old [9] Both versions should be given in 2 doses, 8 weeks apart. In addition to the Pfizer vaccine, Anvisa also approved the use of Coronavac in children between 6 and 17 years old with comorbidities, in a version equal to adults [10], with two doses administered at 28 days apart.

In Portugal, the Pfizer vaccine is approved for use in children from 5 years old [7], with a pediatric version up to 11 years old and a adult from 12 years old. Doses should be administered 6 to 8 weeks apart for the pediatric version, and 21 to 28 days for children over 12 years of age.

Learn more about the COVID vaccine for children.

Possible side effects

According to the WHO [8], the most common side effects of vaccines being used against COVID-19 are:

  • Pain and/or swelling at the injection site;
  • Excessive fatigue;
  • Headache;
  • Muscle pain;
  • Fever and chills;
  • Diarrhea.

These side effects are similar to many other vaccines, including the common flu vaccine, for example. They usually appear in the first 3 days after vaccination and disappear quickly without the need for any specific treatment. Here's how to alleviate the side effects of the COVID-19 vaccine.

There is also a risk that the COVID-19 vaccine could cause severe allergy in some people. While this is a very rare effect, it should be attended to as soon as possible. For this reason, many people need to wait 15 to 30 minutes before being released after vaccination. Still, anyone who shows signs of a severe allergy a few hours or days after vaccination, such as swelling of the face or difficulty breathing, should go to the hospital quickly.

Can the COVID-19 vaccine cause thrombosis?

Although there are some reports of people who developed deep vein thrombosis or pulmonary embolism after getting the COVID-19 vaccination, the risk of these types of complications is considered to be extremely low. The risk of thrombosis by COVID-19 is much higher.

In addition, rare cases of thrombosis associated with thrombocytopenia (decreased platelet count) have been identified 4 to 52 days after adenovirus vaccines (AstraZeneca and Johnson & Johnson). Thus, in the presence of signs and symptoms of thrombocytopenia and thrombosis after vaccination, such as shortness of breath, chest pain, leg pain, blurred vision or bruising, for example, it is important that the doctor is consulted so that treatment can be started.. In these cases, the second dose of the vaccine is not recommended.

Despite the reports, the occurrence of thrombosis due to vaccination is rare and, therefore, vaccination continues to be recommended and is considered safe by the main he alth authorities, such as Anvisa, the European Medicines Agency or the WHO.

Can the vaccine cause Guillain-Barré syndrome?

According to the FDA, in the United States, the Johnson & Johnson vaccine appears to increase the risk of developing Guillain-Barré syndrome in the first 42 days after vaccination. However, these cases are very rare and are not a contraindication for vaccination, which should continue to be done.

Guillain-Barré syndrome mainly affects the muscles and usually causes tingling and weakness in the arms and legs. If this type of symptoms is identified in the first 2 months after vaccination, it is important to consult a doctor or go to the emergency room. See more about Guillain-Barré syndrome, its symptoms and treatment.

Can anyone who has had COVID-19 get the vaccine?

The guideline is that all people can be vaccinated safely, whether or not they have had a previous COVID-19 infection. Although studies indicate that after infection the body develops natural defenses against the virus for at least 90 days, other studies also indicate that the immunity conferred by the vaccine is up to 3 times greater.

In Brazil, the recommendation is that people who have already had COVID-19 be vaccinated after 1 month of infection, while in Portugal this period is 6 months. Complete vaccine immunity is only considered active after all vaccine doses have been administered.

In any case, having been vaccinated or having had a previous infection with COVID-19, it is recommended to continue to adopt individual protection measures, such as wearing a mask, frequent hand washing and social distancing.

Is it safe to get the COVID and flu vaccine together?

According to the Brazilian Ministry of He alth, the application of the COVID-19 and flu vaccine can be applied on the same day, with no interference in the effectiveness of vaccines. However, the recommendation is that the application is made in different muscle groups. If this is not possible, it can be applied to the same muscle group as long as there is a distance of 2.5 cm between each vaccine, so that it is possible to differentiate the adverse effects if they occur.

Who should not get the vaccine

The COVID-19 vaccine should not be given to people with a history of severe allergic reactions to any of the vaccine components. In addition, vaccination should only be carried out after evaluation by a doctor in the case of children under 16 years of age and breast-feeding women.

Patients using immunosuppressants or with autoimmune diseases should also be vaccinated only under the supervision of the treating physician.

During pregnancy, guidelines vary by he alth authorities in each country. In Brazil, vaccination can be done as long as the pregnant woman has a prescription and is over 18 years old, and the administration of Coronavac or Pfizer vaccine is recommended. In Portugal, the vaccination recommendation is for pregnant women over 16 years of age, after 21 weeks of gestation, who have had a morphological ultrasound and more than 14 days after the administration of any other vaccine.

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