HERD IMMUNITY CAN ELIMINATE MEASLES AND POLIO FROM THE PHILIPPINES
What it takes to establish herd immunity
Herd immunity (community immunity; population immunity; social immunity) refers to the protection from a contagious disease that is conferred on an entire population because enough of its people have been immunized against the disease. If a sufficiently high percentage of the population is immunized against a particular virus or bacteria, the microorganism won’t have enough eligible hosts to enable it to perpetuate itself and will eventually die out.
Herd immunity can, in fact, eradicate a disease, as in the case of smallpox, a highly contagious and fatal disease. After an effective smallpox vaccine was developed, a worldwide immunization campaign was undertaken such that by the late 1970s, worldwide herd immunity for the disease was achieved. The last naturally occurring case of smallpox was reported in Oct. 1977 and the World Health Organization (WHO) certified the global eradication of the disease in 1980.
Incidentally, achieving herd immunity is the only way a given population can protect all its members, because there will always be individuals who cannot be immunized such as infants, the elderly, the debilitated, people with severe allergies, pregnant women, or people with compromised immune systems.
Percentage varies
The percentage of the population which must be immunized in order to achieve herd immunity varies for each disease. It can be rather low. For example, in Ghana, a vaccination rate of just 70 percent of the population was enough to eliminate Haemophilus influenza type b (Hib), a bacterium responsible for severe pneumonia and meningitis in children. But for highly contagious diseases, the required figure is much higher. Measles requires 90 to 95 percent immunization coverage to achieve herd immunity while polio requires 80 to 85 percent.
After attaining herd immunity, immunization rates need to be maintained at the required level otherwise, herd immunity can break down leading to an increase in the number of new cases.
Herd immunity for measles and polio in the Philippines
We have achieved some level of success in establishing herd immunity for measles and polio in our country. In 1998, our Department of Health (DOH) embarked on a “Measles Elimination Campaign,” a countrywide program that aimed to eradicate measles in the Philippines by the year 2008. The well-conducted campaign was completed in April 1999 and reached more than 96 percent of the targeted children. The campaign was modestly successful. The number of reported cases of the disease went down from an average of 12,000 per annum before the start of the campaign to just 3,000 in 1999. In succeeding years however, measles vaccination coverage in the country decreased significantly resulting in the accumulation of “susceptibles.”
Thus, measles outbreaks have been periodically occurring since 2009 and they are getting worse. Latest statistics show that over a 12-month period up to June 2019, there have been almost 46,000 measles cases reported in our country.
In the 1990s, our polio immunization campaigns were so successful that the Philippines was certified polio-free by the World Health Organization (WHO) in
Oct. 2000. Sadly, this year, polio has reemerged in our country, as confirmed by the Department of Health (DOH) during the latter part of September when it reported two cases of the disease. And on Nov. 20, the department confirmed three new cases of polio in Mindanao.
Obviously, we have failed to achieve total herd immunity for measles while our herd immunity for polio, which we had achieved, has broken down. These phenomena can be traced to the declining immunization coverage of Filipino children the last few years.
There are many reasons for the decline in immunization coverage of Filipino children including lack of health personnel, difficulty in accessing hardto-reach areas, and lack of time among households to have their children immunized, and, in so far as the last two year are concerned, parent’s fear of vaccines brought about by the Dengvaxia issue.
Herd immunity for measles and polio is an achievable imperative in our country. But to eliminate these two diseases, the government needs the cooperation of all parents.
We appeal to all parents to have their children immunized.
Achieving herd immunity is the only way a given population can protect all its members, because there will always be individuals who cannot be immunized such as infants, the elderly, the debilitated, people with severe allergies, pregnant women, or people with compromised immune systems.