Wednesday May 28, 2008
Pox protection
Chicken pox vaccines can provide up to 20 years immunity and 70% to 90% protection against infection.
By DR MILTON LUM
CHICKEN pox (varicella) is a common infection that affects almost every one. It is caused by the varicella-zoster virus (VZV), which is a DNA virus of the herpes virus family. There is only one type and people are the only reservoir of VZV.
Varicella is very infectious and is found worldwide. VZV is spread by droplets, aerosol or direct contact, and enters the body through the mucosal lining of the eyes and upper airways. The virus multiples in the local lymph nodes and then spreads to the other organs, where it multiplies and then results in the clinical condition. The patient is infectious a few days before the rash appears until the crusts are gone.
The features of varicella include itchy, vesicular eruptions on the skin, fever and tiredness. The rash usually starts on the face and scalp, and spreads to the trunk and extremities. The initial vesicles form crusts that dry up. It usually takes seven to 10 days for all the crusts to disappear.
Consequent to a varicella infection, VZV may reside in sensory nerve roots, from which it may be reactivated later to cause shingles (zoster) infection. About 15% to 20% of those infected by varicella develop zoster in later life.
The complications of VZV infections include secondary bacterial infection, pneumonia, encephalitis, bleeding complications, hepatitis, and arthritis. There may be disfiguring scars from secondary bacterial infection of the vesicles and it may lead to bacterial infection of the blood stream (septicaemia). VZV induced pneumonia is more common in adults. Zoster may sometimes result in neurological sequelae like nerve palsies and impaired vision. About 15% of zoster patients have residual pain or abnormal sensations in the affected dermatomes.
Most women of child-bearing age have immunity to VZV. However, infection in the five days prior to and two days after delivery, may lead to infection of the newborn, in infancy or early childhood.
VZV infection is usually mild in children but more severe in adults. It may be fatal in newborns, senior citizens and those who are immunodeficient. Deaths from varicella in healthy adults are 30 to 40 times more than that of children aged five to nine years. The risk of widespread (disseminated) zoster is increased in patients who have cancers, HIV/AIDS or other conditions in which immunity is deficient.
A natural infection leads to immunity for life in almost all immunocompetent people. Newborn babies of immune mothers are protected by passive immunity, from maternal antibodies, in the first few months of life.
Varicella vaccines
The varicella vaccines are live attenuated vaccines – live disease producing viruses modified in the laboratory so that it stimulates the production of antibodies.
Varicella vaccines can be safely given simultaneously with other vaccines. It is usual to use separate syringes and inject into separate sites. A combined mumps-measles-rubella-varicella (MMRV) vaccine has been available since 2005.
The vaccines are given by injection below the skin surface (subcutaneous) in the upper arm.
The optimal age for varicella vaccination is between the age of one and two years.
The dose recommendation in Japan and many countries is for one dose of the vaccine to be given. The recommendation in the United States is different. One dose is recommended in children below 13 years. Two doses, given four to eight weeks apart, are recommended for adolescents and adults.
Effectiveness and safety
The varicella vaccines are very effective. It is reported from Japan that immunity lasts for at least 10 to 20 years. The Americans report that childhood vaccination provides 70% to 90% protection against infection and more than 95% protection against severe infection.
It is likely that there is some protection against zoster although it has yet to be proven convincingly.
Studies have shown that the vaccines are safe in more than 10 million vaccinees in the United States. Serious adverse effects are very rare.
Side effects
Varicella vaccines are well tolerated and reactions are minimal. It is usually limited to some swelling, redness and thickening (induration) at the injection site for a few hours after the vaccination. There may be a mild varicella rash and fever in less than 5% of vaccinees. This occurs within one to three weeks of vaccination. They are transient and disappear within a few days.
Serious adverse reactions – difficulty breathing, rash, swelling (oedema) of the lips and or larynx – are very rare. They may appear immediately after vaccination or the following day. A bleeding disorder (thrombocytopaenic purpura) occurs very rarely (one in 1 million vaccines).
Vaccination is not advised in those who have fever, have ongoing illness, are pregnant, previously had hypersensitivity reaction to a previous dose of the vaccine, and those who are immunocompromised.
Natural infection
Some parents believe that it is better for their child to get the infection when young since it is usually less severe. Some even organise “chicken pox” parties for this reason. Their belief is that the infection is more natural than the vaccine which is considered “artificial” and that the immunity from the infection will be more permanent than that from the vaccine.
However, when a safe vaccine is available, one has to balance the benefits of natural infection against its potential risks, which include complications that may be severe. There is no way anyone can predict whether a child will develop complications. It should be borne in mind that most cases of varicella with complications occur in previously healthy children.
Surveys of parents and children in many countries have reported that about 70% prefer vaccination to having the natural infection.
Dr Milton Lum is the Chairperson of the Commonwealth Medical Trust. This article is not intended to replace, dictate or define evaluation by a qualified doctor. The views expressed do not represent that of any organisation the writer is associated with.
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