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FAQs on COVID-19 Vaccines

We answer some questions on the efficacy and security of different COVID-19 vaccines

Last update: December 23, 2021

Photo: Thirdman / Pexels

This page provides answer to some specific questions about different approved vaccines against COVID-19.

For general information on COVID-19 vaccines we recommend visiting our FAQs on COVID-19 Vaccines page.

Stay informed on the COVID-19 scientific updates and other global health issues. Click here to sign up for email updates from ISGlobal.

Index

1. Pfizer / BioNTech vaccine

What is the efficacy of the Pfizer / BioNTech vaccine? What protection does it offer against the new viral variants?

In the clinical trials that led to its approval, the efficacy of the Pfizer-BioNTech vaccine (in terms of protecting against symptomatic disease) is 95%. These clinical trials were conducted in controlled conditions (which is why we speak of efficacy and not effectiveness) and when the original viral variant was predominant.

Recent real-world data show waning of antibody levels over time. Antibody levels among people vaccinated with this vaccine seem to stay at higher levels compared to those vaccinated with adenoviral vaccines (AstraZeneca and Janssen) but lower levels compared to those vaccinated with Moderna. In consequence, a 20-30% decrease of its effectiveness against symptomatic disease has been observed. Against omicron, early data indicate that efficacy against symptomatic disease has declined even further (around 20%) but this efficacy is restored with a booster dose (55-80%).

Vaccine protection against hospitalization and death remains high (above 90% efficacy, even with delta). Although there is not sufficient information for omicron yet, the fact that T cells (which protect against severe disease) are mostly unaffected by mutations in this variant suggests that vaccine protection against severe disease and death is maintained.

The UK regularly reports updated data for vaccine effectiveness and impact here .

Last updated on December 23, 2021.

2. Moderna vaccine

What is the efficacy of the Moderna vaccine? What protection does it offer against the new viral variants?

In the clinical trials that led to its approval, the efficacy of the Moderna vaccine (in terms of protecting against symptomatic disease) is 94%. These clinical trials were conducted in controlled conditions (which is why we speak of efficacy and not effectiveness) and when the original viral variant was predominant.

More recent data show waning of antibody levels over time. Even so, antibody levels among those vaccinated with Moderna appear to remain higher compared to the other vaccines.

Consequently, a 20-30% decrease in efficacy against symptomatic disease has been observed against delta. Early data indicate that protection against symptomatic disease caused by Omicron is further reduced and that a Moderna booster dose considerably increases neutralizing activity against Omicron.

Protection against hospitalization and death remains high (above 90% efficacy, even with delta). Although there are not enough data for Omicron, the fact that T cells (which protect against severe disease) are little affected by mutations in this variant suggests that vaccine protection against severe disease and death remains high.

The UK regularly reports updated data for vaccine effectiveness and impact here .

Last updated on December 23, 2021.

3. Oxford / AstraZeneca vaccine

What is the efficacy of the Oxford / AstraZeneca vaccine? What protection does it offer against the new viral variants?

In the different clinical trials conducted, this vaccine was shown to protect against symptomatic infections with an efficacy ranging between 60% and 90%.

A longer interval (12 weeks) between doses has been shown to protect better than an interval of 4-6 weeks, something which has also been observed for mRNA vaccines.

As with mRNA vaccines, real-world data show a decline in antibody levels over time. Specifically, antibody levels among those vaccinated with AstraZeneca-Oxford appear to decline to lower levels compared to those vaccinated with mRNA vaccines.

Consequently, a 20-30% decrease in efficacy against symptomatic delta disease has been observed. According to early data from the UK, protection against symptomatic disease declines sharply (less than 20%), but increases again with an mRNA booster dose.

In contrast, protection against severe disease and death remains high (over 85% efficacy), even against the delta variant. Although there is not yet sufficient data for omicron, the fact that T-cells (which protect against severe disease) are little affected by mutations in this variant suggests that protection against severe disease and death remains high.

The UK regularly reports updated data for vaccine effectiveness and impact here .

Question last updated on December 23, 2021.

Why did some countries stop vaccinating with Oxford / AstraZeneca? Is there any evidence supporting a link between thrombosis and the vaccine?

After a careful review of the cases and consultation with an independent group of experts, the European Medicines Agency (EMA) reached the conclusion that the unusual thrombosis cases observed after the administration of the Oxford / AstraZeneca vaccine are to be included as a severe side effect of the vaccine.

This unusual combination of thrombosis and low platelet counts seems to result from antibodies elicited by the vaccine, which bind to the platelets and activate them. The condition can be treated with intravenous immunoglobulin administration.

These cases are very rare (estimated at 1 in every 100,000 to 250,000 vaccinated people) and a small percentage of them have been fatal (a total of 222 cases, including 18 deaths, among 34 million people vaccinated, as of April 4, 2021).

Therefore, the EMA continues to consider that the benefits of the vaccine largely outweigh the risks.

 

Last updated on November 23, 2021.

What age groups should receive the Oxford / AstraZeneca vaccine?

The EMA recommends the vaccine for all adults aged 18 years onwards.

In Europe, some countries decided not to use it in people above 55 years of age because the clinical trials had not recruited enough people in this age group. However, the results obtained in the UK, where a considerable percentage of elderly people have been vaccinated, as well as the clinical trial conducted in the US, confirm that the vaccine is highly effective in people above 55 years of age (around 85% protection against symptoms and up to 100% protection against hospitalization and death). Therefore, there is no reason to set a maximum age limit for this vaccine.

With the confirmation of the association between rare thrombotic events and the vaccine, some countries have decided to recommend against the use of the vaccine in the younger population. Thus, the UK will offer an alternative option for people under 30 years of age. Spain has decided to be even more cautious and limit its use to those above 60 years of age.

Is it safe to combine a first dose of AstraZeneca with a second dose of Pfizer?

Yes. Several studies demonstrate that combining different vaccines (for example, a first dose of AstraZeneca and a second dose of mRNA vaccine) is safe and effective.

 

Last updated on November 23, 2021. 

4. Janssen vaccine (Johnson & Johnson)

What is the efficacy of the Janssen (Johnson & Johnson) vaccine? What protection does it offer against the new viral variants?

The clinical trial results for the one-dose Janssen vaccine (Johnson & Johnson) showed an efficacy against symptomatic disease of 72% in the US, 66% in Mexico, and 57% in South Africa.

One of the phase 3 trials for this vaccine took place in South Africa, at a time when the beta variant, which is believed can partially escape immunity, was dominant. The efficacy against symptomatic infections caused by this variant was indeed lower than that observed in the US trial (57% versus 72%). However, it was highly effective in protecting against COVID-19 hospitalisations and deaths (above 90%).

Recent data show that a second dose 2-6 months after the first dose further increases the vaccine’s efficacy against symptomatic disease. The US and Europe have recommended that all people vaccinated with the Johnson&Johnson vaccine receive a mRNA booster dose 2-6 months after.

Last updated on November 23, 2021. 

Is there a link between the Janssen (Johnson & Johnson) vaccine and the cases of thrombosis?

After a careful review of the cases and consultation with an independent group of experts, the European Medicines Agency (EMA) reached the conclusion that the unusual thrombosis cases observed after the administration of the Johnson & Johnson vaccine are to be included as a severe side effect of the vaccine.

As with the AstraZeneca-Oxford vaccine, this unusual combination of thrombosis and low platelet counts seems to result from the production of antibodies which bind to the platelets and activate them. The condition can be treated with intravenous immunoglobulin administration.

These cases are very rare (estimated at 1 in every 100,000 to 250,000 vaccinated people) and can be treated if detected promptly.

Last updated on November 23, 2021. 

5. Novavax Vaccine

What does the Novavax vaccine contain?

In contrast to the mRNA or adenoviral vector vaccines, which provide our cells with the instructions for producing the Spike protein, the Novavax vaccine contains the protein itself. It consists of nanoparticles made of the whole Spike protein of SARS-CoV-2, which is produced in insect cells. In order to boost the immune response, these protein nanoparticles are mixed with an adjuvant developed by the company called Matrix-M, a soap-like chemical which is derived from the bark of a tree.

The vaccine can be stored at 2-8 C. It consists of two doses, given 3 weeks apart.

 

Last updated on December 23, 2021.

 

What is the efficacy of the Novavax vaccine? What protection does it offer against the new viral variants?

According to the Phase 3 results announced by the company, the vaccine provides 100% protection against moderate and severe disease, and has an overall efficacy of 90%. These trials were performed at a moment when the alpha variant was dominant. There is still no information regarding efficacy against delta, but it is expected to be slightly lower.

There is no information yet on efficacy against the delta or omicron variants. It is expected to be lower against symptomatic infection but remain high against severe disease.

Results from South Africa showed a lower efficacy against symptomatic disease by beta (51%) but good protection against severe disease and death.

 

Last updated on December 23, 2021.

 

Stay informed on the COVID-19 scientific updates and other global health issues. Click here to sign up for email updates from ISGlobal.

 

6. Treatments

What treatments work for COVID-19?

Finding or developing effective COVID-19 treatments has taken longer than developing the first safe and effective vaccines. The first strategy was to find already existing (repurposed) drugs that were capable of treating COVID-19. The advantage of this strategy is that it is faster (no need to do phase 1 and phase 2 trials). The second strategy is developing new, virus-specific drugs. It requires more time (pharmacokinetics, toxicity and dosing studies must be performed before starting clinical trials), but is bound to be more effective.

Two types of drugs have shown to be effective for COVID-19, depending on the severity of the disease: those that target the virus, which are more effective in the early and middle stages of the disease, where viral replication occurs; and anti-inflammatory drugs that target the excessive inflammation occurring in more advanced stages of the disease.

 

Anti-inflammatory drugs

To date, the only repurposed drug that is clearly effective in reducing mortality among critically ill patients (on oxygen or mechanical ventilation) is dexamethasone (and other anti-inflammatory corticosteroids).

Another anti-inflammatory drug shown to have a beneficial effect in patients with severe COVID-19 is an antibody that targets the inflammatory cytokine IL-6 and is normally used for rheumatoid arthritis (tocilizumab or sarilumab).

 

Antiviral treatments

  • Monoclonal antibodies: Several monoclonal antibodies have been developed and tested. These antibodies bind to the Spike protein of SARS-CoV-2 and inhibit entry into the cell. Their use is recommended in high-risk patients in early stages of the disease, when they have not yet developed antibodies of their own. The disadvantage of these drugs is that they have to be administered intravenously (and therefore in a healthcare setting). In addition, they are not widely available due to their high cost. Different combinations of monoclonal antibodies have been approved for use to treat COVID-19, including those made by Roche, Lilly and Regeneron.
  • Remdesivir: This antiviral drug, originally developed for Ebola disease, has shown some benefit in patients with pneumonia requiring supplemental oxygen.
  • Molnupiravir*:This antiviral drug developed by Merck reduced by 50% the risk of hospitalisation when given within the first days after symptoms appear, according to preliminary results from the clinical trial. It interferes with viral replication by introducing a great number of mutations. The great advantage is that it is an oral pill and can be taken at home. In addition, the company has signed a deal allowing the drug to be made and sold cheaply in low-income countries.
  • Paxlovid*: According to Pfizer, this antiviral drug (also given as an oral pill) reduced the risk of hospitalization or death by 89% in high-risk adults with COVID-19 who were treated within three days of symptom onset. The drug blocks the activity of an enzyme (the SARS-CoV-2 3CL protease) which is required for viral replication. To increase its effect, it was given in combination with ritonavir, an antiretroviral medication normally used for HIV. The company has also signed a deal allowing the drug to be made and sold cheaply in low-income countries.

*Likely to be reviewed and approved shortly by the EMA.

Last updated on November 24, 2021.

 

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