Hepatitis A - Epidemiology

Hepatitis A is one of the most common infectious diseases in humans. In terms of the number of registered cases, hepatitis A ranks third after acute respiratory viral infections and infectious gastrointestinal diseases. Hepatitis A occurs on all continents and in all countries, and the incidence rate depends on sanitary and hygienic conditions and the cultural level of the population. According to WHO, the highest incidence rate is registered in developing countries in Asia, Africa and Latin America. In Western European countries, the incidence rate of hepatitis A is less than 20-30 per 100 thousand people, while in the countries of South-Eastern Europe and the Middle East it ranges from 200 to 500, and in Asian and African countries it increases to 1000 or more per 100 thousand people.

In Russia, the incidence rate fluctuates widely. In recent years, it has been characterized by a rapid decline, which can be explained by the pronounced periodicity inherent in this infection and the structural changes occurring in the Russian population (fall in the birth rate, liquidation of nurseries, pioneer camps, etc.).

The widespread prevalence of hepatitis A is evidenced by the results of virological and serological examinations of various groups (primarily donors) in some cities of Russia, as well as in countries of Europe, America and Asia.

Hepatitis A affects mainly children. In this sense, this infection is rightly called a childhood disease. The proportion of children under 14 in the total incidence of hepatitis A, according to international statistics, is 60% and higher. To date, most registered epidemics and epidemic outbreaks occur in childhood. In all countries, including ours, the most affected age is from 3 to 7 years. This is especially evident in the CIS regions with high incidence. For example, in Uzbekistan, the incidence of preschool children is many times higher than the incidence in other age groups. The same pattern can be observed in Turkmenistan, Tuva, Tajikistan and other regions with high rates of hepatitis A. In places with relatively low incidence, the largest number of patients are registered in older groups of children - 12-14 years old. However, the shift in age-related incidence towards the older population cannot be considered a rule; rather, it is a temporary exception, followed by an increase in incidence among children aged 3–7 years, who are the most vulnerable group for hepatitis A.

Children of the first year of life practically do not get sick with hepatitis A or get sick very rarely. Among 120 children of the first year of life observed by us, who were hospitalized for viral hepatitis, hepatitis B was documented in 40% of cases, hepatitis C in 30, cytomegalovirus hepatitis in 10, hepatitis A only in 7 cases, and hepatitis remained undeciphered in 13% of cases. The low incidence of hepatitis A in children of the first year of life is explained by the presence of transplacental immunity received by the child from the mother, the nature of their nutrition, as well as limited contacts of such children. Theoretically, of course, it can be assumed that children of the first year of life can get sick with hepatitis A, especially if they have not received specific antibodies from the mother or they have already lost these antibodies. Such a situation can be observed in cases where the mother is seronegative with respect to anti-HAV and/or the pregnancy ended in the birth of a deeply premature child. However, although such a combination of circumstances is possible, it apparently occurs rarely, since almost the entire adult population of our country is seropositive for anti-HAV.

Transplacental antibodies in children of the first year of life undergo complete catabolism by 8-12 months, and from this age children become susceptible to the hepatitis A virus. They begin to acquire active immunity, which, naturally, is possible only in the case of their infection with the hepatitis A virus. This process occurs differently in different territories, it is determined by the level of morbidity, the sanitary condition of the population, overcrowding and other factors. For example, American authors note that in Pennsylvania (USA) children have practically no antibodies to the hepatitis A virus, while in Costa Rica half of the examined children already have these antibodies at 2 years old, in Texas (USA) anti-HAV were found in 13% of children under 5 years old, in China - in 42, in Nigeria - in 60, in Australia - in 4%.

With increasing age, the number of people seropositive for antibodies to the hepatitis A virus increases rapidly.

Frequency of detection of anti-HAV in different age groups of healthy population of some countries and territories (%)

Country	Age, years
	10-19	10-29	30-39	40-49	50 and more
Sweden	1	3	9	25	36
Norway	4	5	11	65	58
Speyshria	6	12	30	54	61
Netherlands	7	36	64	77	74
France	25	53	71	87	82
Germany	14	Zi	66	84	94
Greece	68	83	89	88	89
Belgium		64	88	89	91
Former Yugoslavia	95	99	95	98	93
Israel	93	83	95	98	98
Senegal	100	91	67	67	59
Taiwan	95	89	90	83	83
USA	10	23	40	44	63
China	78	87	87	81	78
Nigeria	-	95	99	97	-

As can be seen from the presented data, in most countries the highest percentage of people seropositive for anti-HAV is observed at the age of 50 years and older. The only exceptions are Taiwan, Senegal, Israel, the countries of the former Yugoslavia and partly China, where the number of people with anti-HAV reaches its maximum already at the age of 10-19 years, which may indicate an epidemic problem with hepatitis A in these countries. However, such a conclusion cannot be considered absolutely reliable, since these data are mainly selective and they do not necessarily reflect the incidence of hepatitis A in the country as a whole.

On the other hand, the decrease in the number of people with anti-NAU with increasing age, as in Senegal, can be explained by a decrease in the antibody titer in older age groups. The low percentage of seropositive people aged 10-19 in highly developed countries, such as Sweden, Norway, Switzerland, the Netherlands, Germany, France and the USA, deserves attention, which undoubtedly reflects the high socio-hygienic standard of living in these countries.

The noted patterns in the immune layer in relation to hepatitis A are generally characteristic of our country. As studies in 1999 showed, in Moscow children aged 5-6 years, antibodies to the hepatitis A virus were detected in the blood serum in 50% of cases, and in 11-12 years - in 90%.

In Central Asia and other areas with high incidence of hepatitis A, immunity is acquired at an earlier age, and by the age of 10-15, almost everyone becomes immune.

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