Investigating a population in southern Sweden, a new study has found that a single dose of the vaccine is only about 42% effective in preventing coronavirus disease 2019 (COVID-19). But seven days after the second dose, the vaccine can prevent COVID-19 in 86% of the population.
Several vaccines against COVID-19 have now been approved in different countries around the world. In the European Union, four vaccines have been approved for use. Two of these are the Moderna and Pfizer-BioNTech mRNA vaccines.
In southern Sweden, in Skåne county, vaccination began in late December 2020, starting with nursing home workers and healthcare workers. Researchers from Lund University and Skåne University Hospital evaluated the effectiveness of the Pfizer-BioNTech BNT162b2 vaccine in preventing COVID-19. Their results were published in the medRxiv* preprint server.
Testing vaccine effectiveness
The team used the data from different registers kept in Skåne county that record individual-level data on residency, vaccination, and COVID-19 status. The study included all county residents aged 18-64, and each was followed until 28 February 2021.
Of the total of 805,741 study participants, about 3% had been vaccinated by the BNT16b2 mRNA vaccine by 28 February 2021. About 80% of the vaccinated individuals were women, and the median age of the vaccinated set of people was 47 years.
Two weeks after the first dose, the vaccine was only about 42% effective. But it was 86% effective in preventing COVID-19 in people who had no prior infection seven days after the second dose for the period 15-28 February 2021. There was no difference in vaccine effectiveness between males and females.
People who had been infected before showed 91% effectiveness among those un-vaccinated with a prior positive test. The effectiveness was still high when people had been infected more than three months before vaccination.
The results suggest the vaccine began to work about a week after the first dose, but its effect was low after a single dose.
Second dose necessary for adequate protection
Thus, the study confirmed the effectiveness of the BNT16b2 vaccine among people in the working-age group in a real-world setting. The prevalence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in this region was one of the highest in Europe, with 300-900 new cases recorded every week.
One limitation of the study was the short follow-up time and lack of data to evaluate the effect of disease severity and hospitalization. During the last follow-up period at the end of February 2021, up to 50% of the cases had the B.1.1.7 variant. In addition, there was a lack of information on disease history and other underlying conditions, which prevented a more detailed analysis and correlation.
The study found lower vaccine effectiveness of about 60% within seven days of the second dose. Studies in other countries have found slightly higher effectiveness within a week of the second dose, while a study in Israel showed a similar result as this study.
As our results suggest that vaccine effectiveness may not be satisfactory until seven days after the second dose, it is prudent to inform the public about the importance of maintaining social distancing and complying with other recommendations until full vaccine effect can be expected,” write the authors.
This will be particularly important in regions that still have a high prevalence of the virus.
Vaccinating individuals with prior infection provides strong protection, so this should also be taken into consideration when determining priorities for vaccination. Vaccine effectiveness should be continued to be monitored over the long term to compare the effectiveness of different vaccines and also to look for any adverse effects.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Björk J. et al. (2021) Effectiveness of the BNT162b2 vaccine in preventing COVID-19 in the working age population - first results from a cohort study in Southern Sweden. medRxiv. https://doi.org/10.1101/2021.04.20.21254636, https://www.medrxiv.org/content/10.1101/2021.04.20.21254636v1