World Health Organization (WHO) estimates that on average 2 billion people suffer from infections every year; 17 million of them die [1]. Vaccination is the most effective and economically sound tool for controlling infections, given that it is promoted worldwide and the involved healthcare workers are committed to its goals. Due to vaccination, approximately 2 to 3 million infection-related deaths were prevented in 2016; but better coverage could have saved another 1.5 million lives [1].

Good vaccination coverage is one of the key indicators of a successful vaccination strategy. To ensure protection against the majority of vaccine-preventable infections, WHO expects vaccination coverage to be at least 95 % among children and 90 % among adults. Officially [2], Russia has already achieved these high goals and now only maintains the recommended rates. Plummeting incidence and mortality related to vaccine-preventable infections indicate that the chosen vaccination strategy is effective.

Inspiring results of vaccination drove WHO to announce a new era in healthcare: smallpox had been eliminated by 1980, no incident cases of polio have been reported in Russia since 2002. Although measles and rubella had not been eradicated by 2010, as planned, there is hope for this to happen in the future. In 2000, the USA declared elimination of measles [3]; however, in 2010 the situation changed: measles incidence rates started to increase in both Americas, Europe, Africa, and the former Soviet Union, including Russia [4]. Therefore, the deadline for measles elimination was extended to 2015 and then beyond 2020.

Growing measles incidence was alarming, prompting the medical community to question the reliability of key indicators of vaccination effectiveness. A few new factors were discovered negatively affecting vaccination outcome, including weak cold chains, unsubstantiated contraindications, increasing vaccine hesitancy among parents and failure to adhere to vaccination schedules. Unfortunately, vaccine hesitancy is becoming a pressing issue in Russia. Polibin et al. [5] indicate that in Russia the population tends to avoid any preventive measures, including vaccination. Parent surveys show that only 80 % of infants below 2 years of age are vaccinated.

Many authors believe that the troubling situation with measles resulted from reduced herd immunity, i. e. the proportion of people susceptible to infection [6, 7, 8]. The only way to measure it is to resort to serological surveillance [8, 9]. The proportion of seronegative (or susceptible) individuals is calculated using laboratory serum tests. Seronegativity rates of < 7 % and 15 % for measles and mumps, respectively, are epidemiological health thresholds.

In Russia, serological surveillance is not perfect. One of its downsides is the inability to compare results of studies of serum samples collected in different years, accounting for the rapid evolution of laboratory diagnostic techniques and standards. Besides, serological studies ignore professional occupation of the participants (for example, vaccination requirements for decreed population groups are stricter) and their general health (infections affect humoral immunity). Serum banks may provide a solution here.

A number of Russian researchers argue that healthcare workers contribute to the risk of infection outbreaks in healthcare facilities [10, :lit_11]. Healthcare personnel is regarded by the law as a decreed population group; members of this group must be vaccinated against measles before turning 55 (or 35, in the case of rubella and mumps) [12]. These people are not only at risk of getting infected; they also indirectly signal the epidemiological status of the population.

This study aimed to evaluate the actual susceptibility of the Russian population to measles, rubella and mumps by analyzing the samples from the Serum bank of N. F. Gamaleya Federal Research Centre for Epidemiology and Microbiology, Moscow.

METHODS

We studied 866 serum samples, including:– 293 samples collected in 2011 from healthcare workers

– 293 samples collected in 2011 from healthcare workers residing in the Central Federal District (CFD);
– 117 samples collected in 2017 from healthcare workers residing in CFD;
– 220 samples collected in 2016–2017 from contracted soldiers (military recruits) serving in Moscow region;
– 236 samples collected in 2016 from seemingly healthy individuals residing in Moscow and Moscow region.

In 2011 the situation with measles significantly deteriorated in Russia, and in 2017 extra preventive measures were taken supported by the State, including immunization of previously unvaccinated population groups and increasing the vaccination age for medical workers from 35 to 55 years [8, 12]. Outbreaks of measles, rubella and mumps are, however, still reported in the armed forces, demonstrating a problem of a closed susceptible group [13].

In our study, vaccination histories of sample donors were unknown, no documented evidence on their vaccination status was available.

The study was designed and conducted in compliance with the guidelines of the Russian Federal Service for Surveillance on Consumer Rights Protection and Human Wellbeing [9]. Serum samples were studied using ELISA and a number of testing kits, including Vecto-Measles IgG, Vecto-Rubella IgG, Vecto-Mumps IgG (by Vector-Best, Russia), according to the manufacturer’s protocol.

Statistical analysis was performed in Microsoft Excel and Statistica 6.0 (StatSoft, USA). Significance of differences between the groups was calculated using the chi squared test and Yates’ correction. Mean values (m) were calculated for all studied parameters; a 95 % confidence interval was calculated using the Clopper–Pearson method.

RESULTS

In the course of our study, we evaluated immunity to vaccine-preventable infections among different population groups. Results are shown in tab. 1.

The table shows that all studied groups were protected against rubella: the proportion of seronegative samples did not exceed the 7 % threshold.

A considerable proportion of study participants were susceptible to measles, military recruits being the most vulnerable group, where the proportion of seronegative samples reached the maximum of 25.5 %, which is 3.6 times higher than the epidemic threshold. Among medical workers, susceptible individuals accounted for 7.7 %.

Among the healthy donors residing in Moscow and Moscow region, the proportion of susceptible individuals was quite large (21.2 %). This people may contribute to the spread of measles, forming epidemic foci, especially in the armed forces, should the pathogen find its way into the region. Thus, the current situation with measles in Moscow and Moscow region can be characterized as unstable.