The main uncertainty today about the Covid-19 pandemic is how well and for how long resistance remains after infection and/or vaccination.

The report of the European Centre for Disease Prevention and Control (ECDC) of 18 July 2022 showed that a third dose of vaccination (first booster dose) does substantially reduce the risk of Covid-19 infections, but the protection against serious illness is better. The protection is, however, subdued during the first three months after the vaccination and varies according to the strain of virus but the protection is lower against the imidacrine strain than for other strains. A fourth vaccination dose, however, greatly improves protection, but there is not enough experience yet to say how long it lasts.

The CDC also monitors how well and for how long protection after the first infection with Covid-19 lasts. In Iceland, 200,397 Covid-19 infections have been officially diagnosed since the beginning of the epidemic, of which 5,116 have been detected two times and 19 three times. Thus, the rate of retransmission as a whole represents 2,6 % of all cases, but a higher rate may be expected because not all those diagnosed with retransmission, e.g. will undergo a home test for official tests.

A closer look at the number of re-infections revealed that of the 30,487 detected for the first time in 2020 and 2021, 4,026 were re-infected in 2022 after the imidazon wave began or 13.2%. Of the 169,069 cases that became infected for the first time in 2022, 841 have become re-infected (0.5%). It should be kept in mind that internationally defined re-infection is a new infection that is detected 60 days or later after previous infection.

Thus, it is clear that the risk of re-infection depends on time since previous infection, reflecting decreasing levels of protection over time. Also, new strains of the virus seem to be able to escape resistance from previous strains more easily. The number of cases of re-infection has increased significantly in the last two months and in the last few weeks it has been 20% of the daily number of cases. The increase is linked to an increase in BA. 5 strain of the coronavirus, but this strain now accounts for 80% of all infections in Iceland. Foreign studies have also shown that BA. 5 variant escapes more than other strains from immunity caused by previous infections.

The coming weeks and months will determine how common re-infection with new strains of the coronavirus will occur. This knowledge will weigh heavily in decision-making on revaccination next autumn/winter.

The Chief Epidemiologist