“The bad news is that the protection against infection seems to be diminished by seven months after the second dose of vaccine,” said Peter Nordstrom, a professor at Umea University in Sweden.
“The good news, however, is that the protection against a severe infection that leads to hospitalisation or death seems to be better maintained. Vaccination is therefore very wise and important,” Nordstrom said.
The study is a nationwide, observational study based on registry-data from the Public Health Agency of Sweden, the National Board of Health and Welfare, and Statistics Sweden.
The main analysis included almost 1.7 million individuals, and the results were confirmed in an even larger population of almost 4 million individuals.
The results showed that protection against infection of any severity waned progressively following the peak which occurred a month after the second dose.
Six months after vaccination, the remaining protection against infection was 29 per cent from two doses of Pfizer, and 59 per cent from two doses of Moderna vaccine.
There was no remaining protection from a month and onwards for AstraZeneca, the researchers said.
With respect to infections that were severe enough to warrant a hospital stay, or where the individual died within 30 days of confirmed infection, the protection was better maintained, they said.
The study found that protection against severe disease was 89 per cent after one month and 64 per cent from four months and onwards during the rest of the maximum follow-up of nine months.
“The results underscore and support the decision to offer a third dose,” said Marcel Ballin, doctoral student at Umea University and co-author of the study.
“In particular, the results show that it was correct to prioritise the oldest and frailest individuals,” Ballin said.
Prior to this study, a few observational studies and follow-up studies of the clinical trials have investigated waning vaccine protection in other countries.
However, these have mostly covered the initial four to six months, and for the Pfizer vaccine.
“What this study contributes with is the longer follow-up time and the fact that we were able to explore how well the protection is maintained according to different types of vaccines,” said Anna Nordstrom, adjunct professor at Umea University, and co-author of the study.
“The strengths are that we have been able to do this in a real-world setting based on a population-based sample of the total population of Sweden. This increases the possibility to generalise the results to other countries with similar population structure as in Sweden,” Nordstrom added.