by Dr. Milton Lum
Tuberculosis (TB) is caused by the bacterium, Mycobacterium tuberculosis. It is spread by air droplets when an infectious TB patient coughs or sneezes. The bacterium may be killed by the body's immune system, multiply and cause primary TB, remain dormant and remain asymptomatic, or proliferate after a latency period.
An infected person may not develop the disease. Such people are not infectious, that is, they do not spread TB to others. TB usually affects the lungs but can also affect other parts of the body like the bone, joints, genito-urinary tract, gastro-intestinal tract and brain.
TB is the most common infectious disease worldwide and the number one killer. The World Health Organisation (WHO) estimates that about 2 billion people have latent TB and the annual global deaths are about 3 million.
According to the Health Ministry, the incidence of TB in 2005 was 61.2 cases per 100,000 population and the mortality rate, 5.5 per 100,000 population. The comparative data for dengue, the next most common infectious disease here, was 60.71 and 0 per 100,000 population.
TB affects all ages, especially children below the age of 3, older people and those with impaired immunity. TB may present in many ways and diagnosis may be difficult. The complications of TB include respiratory problems, infertility, limb disablements, incapacity and death.
The incidence of TB has been decreasing until about a decade ago when it began to increase due to the increase in the incidence of HIV/AIDS and multi-drug resistance to TB.
The Bacille Calmette-Guerin (BCG) vaccine is a live attenuated strain of Mycobacterium bovis. It has been in use since World War I and provides protection against TB meningitis and disseminated TB in infants and children, both of which are fatal, if untreated. However, it does not prevent primary infection and reactivation of latent TB of the lungs, which is the main source of the spread of TB in the general population. As such, BCG vaccination has primarily no impact on the transmission of TB.
Despite its shortcomings, the BCG vaccine is an important component of TB control as its use has saved many lives.
The vaccine requires one injection into the skin over the shoulder tip of the newborn child. The injection usually leads to a minor local reaction of redness, swelling and tenderness. This is often followed by the formation of a small ulcer at the injection site. The extent of this reaction is related to the age of the recipient, dose and strain of the vaccine, and the skills of the health care professional who does the vaccination.
After a few months, a small scar forms. The presence of a typical scar is an indicator of previous BCG vaccination but not that of protection against TB. If a scar is absent in older children, BCG vaccination is given.
Effectiveness and Safety
The BCG vaccine is effective. The average protection against TB meningitis and disseminated TB is about 80%. However, the protection against TB of the lungs varies from 0 to 80%, with a lower protective effect in tropical countries. The duration of protection after BCG vaccination of the newborn is not well known. It is believed to decline gradually after 10 to 20 years.
BCG vaccination is safe as complications are rare. The incidence of death after vaccination has involved accidental vaccination of individuals with severely compromised cellular immunity. BCG vaccine is not given to persons who have impaired immunity, are receiving immunosuppressive medicines or who are pregnant.
Adverse events are usually related to infection by the live attenuated bacteria, and errors in achieving intra-dermal inoculation. They are mild or severe. The most common adverse effect is a local abscess. More extensive and prolonged local reactions do occur.
However, secondary infections of the injection site are uncommon. Swelling of the lymph glands in the armpit and neck may occur. It heals spontaneously in most instances without any treatment. Antibiotics may be required occasionally.
Systemic infection by BCG is a recognised but rare adverse event. It is only seen in children with severe immune deficiencies. Other rare adverse events include BCG-induced bone infections such as osteitis and osteomyelitis.
Premature infants born less than 33 weeks of pregnancy are less likely to develop the BCG scar. As such, premature infants are not given the BCG vaccine until they are at least 34 weeks, and with a body weight more than 1.8kg to 2kg. BCG vaccine is not given to children with symptoms of HIV. Babies born to HIV-positive mothers are not likely to have symptoms of HIV. The WHO recommendation is that they be given the BCG vaccine.
Many new vaccines against TB are being assessed in current research. The primary objectives of these evaluations are protection against infection in those who have never been infected, prevention of reactivation of latent infection and prevention of relapse in TB patients.
The BCG vaccine is safe and does not interfere with other vaccines given simultaneously. It provides about 80% protection against TB meningitis and disseminated TB. It does not, however, impact significantly on the diseases burden of TB. This together with the growing incidence of HIV/AIDS and the increasing multi-drug resistance, threaten current TB control strategies.
Until improved vaccines are available, control of TB is dependent on the currently available tools of early detection, directly observed therapy, and appropriate preventive and infection control measures.
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