Some residents of Vilnius and Kaunas will soon find invitations to take part in an important new research in their mailboxes. In a joint project between the Lithuanian University of Health Sciences (LSMU) and Vilnius University (VU), researchers will assess the extent of SARS-CoV-2 infection and the level of antibodies against this infection.
The COVID-19 pandemic has been one of the greatest public health challenges of our time, claiming close to 7 million lives worldwide and 9,500 deaths in Lithuania.
"Today, the SARS-CoV-2 causative agent of COVID-19 is no longer as dangerous and the majority of the population is vaccinated and feels safe. However, there are few SARS-CoV-2 sero-monitoring studies in Lithuania and worldwide, which are essential for the future vaccination debate," said Dr Martynas Simanavičius, one of the researchers conducting the study. 
The majority of the adult population received a booster dose of the vaccine a year ago. Research shows that virus-specific antibodies persist for 13 months after SARS-CoV-2 infection, and that the vaccine's effect is enhanced by previous infection. However, very little information is available on vaccine response in the COVID-19 exposed and unexposed groups.
To assess the true extent of infection and the effectiveness of vaccination, serological tests are usually performed to detect specific antibodies to viral pathogens.
The first such large-scale sero-epidemiological study in Lithuania was carried out in 2020 to assess the extent to which the population was infected with COVID-19, as not all asymptomatic cases were included in the official statistics. The 2020 survey used rapid serological tests available at the time, which could only answer the question of whether or not antibodies to SARS-CoV-2 had been formed.
LSMU and VU are currently conducting a joint project funded by the Research Council of Lithuania entitled "Investigation of the prevalence of SARS-CoV-2 infection in Lithuania using serological methods". This study will help to estimate the true population prevalence of SARS-CoV-2 infection over the last six months. It will also assess the level of antibodies against SARS-CoV-2 in the vaccinated population.
According to Dr Indrė Kučinskaitė-Kodzė, who is involved in the project, the study will use an innovative, Lithuanian microgel-based assay to simultaneously quantify antibodies to SARS-CoV-2 nucleocapsid (anti-N IgG) and spike (anti-S IgG) proteins from a small volume of capillary blood sample. 
What is their significance?
As Prof. Astra Vitkauskienė explained, anti-S IgG proteins are found in the blood of both vaccinated and vaccinees, while anti-N IgG proteins are only found in the blood of vaccinees (since the N protein is not present in vaccines). It is important to assess whether specific antibodies in vaccinated persons are due to the vaccine or to exposure to the virus itself, even though the disease may not have manifested itself clinically and the presence of the virus in the nasopharynx was not confirmed by PCR. The detection of the virus in the nasopharynx by PCR is shorter than the detection of anti-N and anti-S IgG antibodies in the blood by serology.
"We hope that the results will provide very important indicators for the Lithuanian population - how long antibodies persist, what is the actual proportion of SARS-CoV-2 infected persons in the population, and will add to the discussion on the need for revaccination," said Prof. Mindaugas Stankūnas, the leader of the study.
Invitations to take part in the study will soon be in the mailboxes of randomly selected residents of Vilnius and Kaunas cities and districts. The invitations will contain detailed information on the progress of the study and contacts.
This study has been authorised by the Lithuanian Bioethics Committee.
Photo credits:
Dr Martynas Simanavičius and Dr Indrė Kučinskaitė-Kodzė in photos by Justinas Auškelis (VU)