Study of all fully vaccinated people in Belgium

A study of all fully vaccinated people in Belgium assessed risk factors associated with the development of COVID-19 after vaccination. The research, which is presented at this year’s European Congress of Clinical Microbiology and Infectious Diseases (ECCMID) in Lisbon, Portugal, (April 23-26) and published in the journal Virus, found that breakthrough infections were more common in people who received viral vector vaccines than mRNA vaccines. Those who had already had Covid had a lower risk of breakthrough infections.

More than 80% of adults in Belgium were fully vaccinated (received two doses) by August 11, 2021.

Four different vaccines were used – Pfizer/BioNTech (BNT162b2), Moderna (mRNA-1273), Oxford/AstraZeneca (ChAdOx1) and Janssen (Ad26.COV2.S).

Dr. Veerle Stouten and his colleagues at Sciensano, Belgium’s National Institute of Public Health, studied the incidence of breakthrough infections – infections that “break through” the protective barrier of vaccination – for each brand of vaccine.

They also studied the risk factors and symptoms associated with breakthrough infections.

This information can be useful when designing immunization programs, including decisions about who would benefit most from a booster dose and when and what type of vaccine to use.

The study included all people in Belgium aged 18 and over who were fully vaccinated against COVID-19 between February 1 and December 5, 2021. The 8,062,000 participants were followed for an average of 150 days , starting 14 days after their second dose.

4.6% of participants (373,070) had a breakthrough infection. However, this figure does not take into account the length of follow-up of each participant. When this is taken into account, the incidence rate is 11.2 per 100 person-years. This means that if 100 participants were followed for a year, 11.2 would be expected to develop a breakthrough infection.

The researchers also looked at factors associated with breakthrough infections. These analyzes were adjusted for age, sex, prior COVID-19 infection, and exposure of a person to the SARS-CoV-2 virus by their occupation or environment.

Vaccination with a viral vector vaccine (Oxford/AstraZeneca or Janssen) was associated with a higher risk of breakthrough infection than an mRNA vaccine (Pfizer/BioNTech or Moderna).

The Oxford/AstraZeneca vaccine was associated with a 68% higher risk of breakthrough infection than the Pfizer/BioNTech vaccine. The Janssen vaccine was associated with a 54% higher risk of breakthrough infection than the Pfizer/BioNTech vaccine.

In contrast, breakthrough infections were 32% less likely to occur in those who received the Moderna vaccine than in those who received the Pfizer/BioNTech vaccine. (The Pfizer/BioNTech vaccine was used for all comparisons.)

The researchers point out, however, that the study was not designed to perform a formal comparison of vaccine effectiveness between brands.

There was a higher incidence of breakthrough infections in younger age groups (18-64) than in older age groups (65-84 or 85+), which could be due to differences in social behavior.

Healthcare workers were 40% less likely to develop a breakthrough infection than other healthcare workers. This could reflect “good protection” of healthcare workers – high vaccination coverage and heavy use of PPE at work.

The analysis also found that those who had already been infected with COVID-19 before vaccination were 77% less likely to have a breakthrough infection than those who had not had Covid before.

Of 216,814 breakthrough infections with available symptom information, the majority (70.1%) were symptomatic, with rhinitis (a runny nose), cough and headache being the most common symptoms.

Among people with a breakthrough infection, those who had had an infection before were 62% less likely to have symptoms than those who had not had Covid before.

The study authors conclude: “We observed an incidence rate of breakthrough infections of 11.2 per 100 person-years based on a large Belgian national cohort of fully vaccinated people. This means that if 100 participants were followed for a year, 11.2 would be expected to develop a breakthrough infection.

Dr Stouten adds: “We have identified risk factors associated with breakthrough infections, such as vaccination with adenoviral vector vaccines, that could help inform future decisions on scale-up booster vaccination strategies. international.

“Furthermore, we observed that hybrid immunity from combined prior infection and vaccination not only reduced the risk of breakthrough infections, but also reduced the risk of having symptoms during a breakthrough infection, highlighting its protective effect.

“The majority of breakthrough infections included in the study occurred during the period when the Delta variant was dominant. We expect to see similar patterns regarding the characteristics of breakthrough infections due to the Omicron variant, but we must continue to monitor breakthrough infections and study their severity and multiple recurrences, as well as the role of emerging variants to confirm this.

Dr Veerle Stouten, Department of Epidemiology and Public Health, Sciensano, Brussels, Belgium E) [email protected]

Alternate contact: Tony Kirby at the ECCMID Media Center. T) +44 7834 385827 E) [email protected]

Notes to Editors:

The authors declare no conflict of interest.

This press release is based on oral presentation L0351 at the annual meeting of the European Congress of Clinical Microbiology and Infectious Diseases (ECCMID). The material was reviewed by the congress selection committee. An article based on this research has just been published in the journal Virus.

As the full paper is provided here, there is no link to the conference summary

For the press release in Spanish, click here

For the press release in Portuguese, click on here


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