Since last Friday, February 3, Statistics Lithuania has included two new indicators in the daily statistics of the COVID-19 pandemic: the number of recurrent secondary and tertiary infections (reinfections). We are probably the first in Europe and one of the first in the world to do so – our British colleagues have also started publishing reinfection data in the beginning of February, slightly earlier the publication of such indicators was started in the United States.

Why "only now"?

The number of recurrent infections (as well as the number of post-vaccination infections) is and has been monitored continuously. Prior to the spread of the Omikron variant, recurrent infections did not exceed 1 per cent. Since January 2022, the number and percentage of recurrent infections has started to grow extremely rapidly, accounting for a significant share of almost 15 per cent of all infections. Therefore, the reinfection rate has been urgently included in the COVID-19 daily report. It is important to keep in mind that the approximate number of re-infections has always been available for our population in open data sets, Table “COVID19 statistika dashboards”: the number of positive diagnostic tests on any day to be compared with the number of new cases on that day.

What is a re-infection?

Apparently, we are all aware that two positive one-person test responses do not necessarily mean two different infections. A person can be sick for several weeks, while the genetic material of the virus can be detected even a few months after the infection. Thus, two positive one-person diagnostic test responses must be separated by a sufficient number of days, a "period of silence", so that to be considered as two different infections.

90-day "silence period"

A 90-day “silence period” is used to calculate recurrent infections, a minimum of such number of days between two infections (i.e. at least this number of days the person has not received a positive diagnostic test response). This interpretation is shared by other countries and international health organizations. The ideal "silence period" should be longer than the period of detection of the virus after infection and shorter than the period between the two adjacent pandemic waves.

How strongly the estimated number of re-infections depends on the choice of the "period of silence":

• if the “silence period” is 30 days, the number of re-infections over January 2022 amounted to 23,957;
• if the “silence period” is 60 days, the number of re-infections over January 2022 amounted to 23,432;
• if the “silence period” is 90 days, the number of re-infections over January 2022 amounted to 22,162.

In other words, the number of re-infections is not very sensitive to the chosen “period of silence”'.

How are other countries behaving?

As already mentioned, only a few countries have started publishing official re-infection rates using the same 90-day “silence period”. For several months, however, the European Center for Disease Prevention and Control (ECDC) has already been talking about the growing need for data on reinfections. It is likely that other countries and the ECDC itself will soon start publishing the number of re-infections.

Please note!

It is not easy to interpret the number of re-infections, as it is necessary to know the size of the population at risk. Without this information, two different days, two different municipalities, or two different indicators (e.g., primary with secondary infections) cannot be compared. For example, 3 tertiary infections a day is it much or little? To do this, you need to know the total number of secondary infections up to that date (because it is not possible to become infected for the third time without having been infected a second time), at least 90 days after the person did not receive a positive diagnostic test response; counting only those persons who were still alive before that date and had not yet acquired a tertiary infection.

The number of primary, secondary and tertiary infections is published daily in the daily report of Statistics Lithuania https://osp.stat.gov.lt/praejusios-paros-covid-19-statistika.  

Total number of secondary infections during the pandemic since March 2020 amounted to 36,709, of which over January 2022, 22,045. Tertiary infections during the pandemic since March 2020 totalled 172, of which over January 2022, 117.

Statistics Lithuania prepares a particularly large number of operational, timely pandemic indicators and has opened all the most important data. We call for their responsible and ethical use. We would also like to share this report with all those who were too quick to create a sensation and express their indignation over the “hidden” data.

For inquiry:
COVID-19 data team
Email covid@stat.gov.lt
FB account https://www.facebook.com/ValstybesDuomenuAnalitika