Full vaccination series provides excellent protection against severe coronavirus disease, also among those receiving two different vaccines

A study conducted by the Finnish Institute for Health and Welfare (THL) shows that after two doses the effectiveness of all the coronavirus vaccines used in Finland is good against coronavirus infection, and excellent against severe COVID-19 that requires hospitalisation. 

The results are also excellent for those vaccinated with two different vaccines (i.e. “mix and match”).

The studied vaccines were the AstraZeneca adenovirus vector vaccine Vaxzevria and the mRNA vaccines – Biontech-Pfizer's Comirnaty and Moderna's Spikevax. The effectiveness was examined in the working-age population in Finland – those aged 16–69 years. 

The results are still preliminary, and they have not yet been subjected to scientific peer review. 

Positive results from mixed and matched vaccine series

The study revealed that after two doses the protective effect of the mRNA vaccines against coronavirus infection was 78 percent in the age group under study. The effectiveness of the adenovirus vector vaccine was also 78 percent and that of the mixed and matched vaccine series 80 percent.

• The confidence intervals of the results are narrow. It is seen to be 95 percent certain that the effectiveness of the mRNA vaccines against infection ranges between 76 and 79 percent. The corresponding confidence interval is 73–83 percent for the adenovirus vector vaccine and 77–83 percent for the mixed and matched vaccine series.

After two doses the vaccine effectiveness against hospitalisation for severe COVID-19 was excellent. The effectiveness of the mRNA vaccines against hospitalisation was 97 percent and that of the adenovirus vector vaccine 93 percent. The effectiveness of the mixed and matched vaccine series was 91 percent. 

•  The corresponding 95 percent confidence intervals were 94–98 percent for the mRNA vaccines, 81–97 percent for the adenovirus vector vaccine, and 80–96 percent for the mixed and matched vaccine series.

“The effectiveness figures that we have estimated for the different vaccine series are similar and their confidence intervals overlap. For this reason, our analysis does not indicate any meaningful differences in the effectiveness of the different products and their combinations”, says Eero Poukka, Medical Specialist at THL.

“It was especially important to confirm that those vaccinated with two different vaccines are as well protected as those who received two doses of the same vaccine. Since so far only a few studies on the effectiveness of the mixed and matched vaccine series have been published internationally, these preliminary results are very valuable”, Poukka adds.

The recipients of the mixed and matched vaccine series were first given the adenovirus vector vaccine, followed by one of the two available mRNA vaccines.

Studies on vaccine effectiveness continue

In the future THL will study the long-term protection afforded by the corona vaccines – that is, the effectiveness in the subsequent months that follow the second vaccination. THL is also currently studying the effectiveness of the vaccines against the Delta variant of the virus.

“Studies conducted in countries such as UK and USA demonstrate that the Delta variant weakens the effectiveness of the vaccines against coronavirus infection, although the vaccine effectiveness against severe COVID-19 remains being excellent”, says THL statistician Ulrike Baum.

“We also study how the extended time interval between doses affects the effectiveness of the vaccines. Preliminary immunological studies have shown that a longer inter-dose interval increases antibody levels, but we still need more information on how the length of this interval influences the effectiveness.”

THL's research on the effectiveness of the vaccines is based on information from health care registers between 26 April – 16 August, 2021. Everyone aged 16 to 69 years living in Finland, or nearly 3.7 million people, was included in the study. Persons who had previously been diagnosed with a coronavirus infection or hospitalised for COVID-19 were excluded from the analysis. The results were adjusted for age, which means that the impact of the unbalanced distribution of age between vaccinated and unvaccinated was considered.

The effectiveness was estimated after at least seven days had passed from receiving the second vaccine dose.

Further information:

Eero Poukka
THL
Medical Specialist
[email protected]

Ulrike Baum (in English)
THL
Statistician
[email protected]