Digest author: Stylianos Loukides, ERS E-Learning Director / 21 March, 2021
As the COVID-19 pandemic continues to spread around the world and an increasing number of the world’s population is infected, a key question that rightly arises is whether COVID-19 disease offers protection against re-infection with SARS-CoV-2.
In 2020, extensive population tests were performed in Denmark by PCR for SARS-CoV-2. Throughout the year, 10.6 million PCR tests were performed on approximately 4 million citizens, representing 69% of the Danish population. Interestingly, the capacity to do PCR testing for SARS-CoV-2 in Denmark increased rapidly over 2020. By the end of the year approximately 10% of the population was tested each week on average.
The analysis did not include those citizens who tested positive for PCR for the first time between the two main waves of COVID-19 (1st wave: March – May 2020, 2nd wave: September – December 2020), as well as those who died before the second wave. During the first wave before June 2020, 533,381 citizens were tested, of whom 11,727 (2.2%) tested positive for PCR. Of these, 525,339 were available for retest on the second wave with a positive rate on the first wave of 2.1%.
It is worth noting that 72 people from this group of citizens (0.65%) again showed a positive PCR test in the second wave of the pandemic in Denmark. In contrast, 16,819 citizens showed a positive PCR test out of 514,271 who had a negative PCR test during the first wave (3.27%). Overall, the individual history of previous COVID-19 infection offered 80.5% protection against re-infection with SARS-CoV-2. In the sub-analyses carried out, the degree of protection was clearly lower (47.1%) among citizens aged 65 and over. However, there were no differences in the degree of protection depending on the sex of the participant (male / female) or the time interval between the first PCR test (3-6 months / 7 months or more).
It is important to state that the test frequency during the second wave was a little higher among those who did not have a positive test result during the first wave. When the sample was restricted to health care personnel some interesting data was retrieved. They had a median of 10 tests done each in 2020, and 658 (4.2%) tested positive during the first wave. Eight (1.2%) of 658 who tested positive in the first wave also tested positive during the second one. A similar protection was also finally revealed reaching 81%.
In conclusion, this study suggests that the risk of re-infection following a previous COVID-19 infection is not negligible, especially in older people over 65 years of age. Therefore, vaccination is especially important even for citizens who have a personal history of COVID-19 infection.
Two important points have been addressed in this study. The first one is the continuous testing as part of epidemiological surveillance. The second one is the vaccination policy irrespective of previous infection. Both will increase the protection against SARS-COV-2.