Influenza H1N1 Update - Aug 09

Influenza H1N1 Update

We now have a significant amount of information about the current H1N1 pandemic. As described in our previous newsletters the symptoms associated with H1N1 are comparable with seasonal influenza. The current mortality is lower.

According to the Centre for Health Protection, as of 8th September there have been 14,867confirmed cases of Swine Influenza in HK of which 69 cases have been severe. There have been 12 deaths. The number of daily cases continues to increase. Hospital admissions show a similar pattern. The reported cases will underestimate the total number cases as mild cases are less likely to be reported. For the same reason, reported cases will also tend to exaggerate the severity of the disease.  As we have previously explained mortality rates early in epidemics are unreliable.

Influenza epidemics occur in waves. These waves will affect different countries and communities at different times. The first wave of infections in Mexico and the USA was followed by waves in Australia and the UK. All these countries are now seeing a decline in incidence. In Hong Kong we are seeing a slow rise in our first wave of infections (see graph)

The sharp fall in cases in hospital cases at the end of June occurred because the government stopped its policy of quarantining cases.  The actual rise in cases has been relatively slow, as you can see when the above data are compared with a graph of cases in the UK

Although seasonal influenza is less prevelant in warmer climates it is also likely that the public health measures taken in Hong Kong, such as closing schools and quarantine of cases have slowed the rate of transmission of the virus. There is a possibility that there will be an increase in cases once schools are back and when the cooler weather returns in the autumn. There is a precedent for the number of cases of flu increasing with school return in a previous pandemic in the UK:

Source: BBC Health Website

As described in our past newsletters understanding the nature of the epidemic helps us as individuals to understand the risks of catching the infection. H1N1 is very infectious and current estimates suggest that around 30% of the population will be infected. Understanding the epidemic also allows governments and health agencies to put strategies in place to minimize the burden of the disease in population terms. Although the disease is relatively mild, social distancing, such as school closures can be effective in flattening the epidemic curve. This not only reduces the risk to individuals but also makes it less likely that large numbers of people will be ill at the same time thus reducing the burden on health systems and key public and private organizations which could be adversely affected by large scale absenteeism. The temporary closure of schools, such as HKIS this week, is a sensible and well validated public health response. It does not indicate that the disease is increasing in severity.

So far the majority of cases have affected younger people and have caused mild illness. Older patients appear to have some background immunity although as with seasonal flu older patients who catch the disease have more severe illness in general. In a typical seasonal influenza epidemic, 90% of the deaths occur in the elderly. In this epidemic most deaths have occurred in individuals less than 50, most have had existing medical problems. It is reasonable to think of this disease as like seasonal influenza but with a lower mortality because older patients are not catching it as easily. Overall most infected individuals experience mild symptoms and drug treatment has not been necessary in the majority of cases. Vaccination is likely to play a role in the future but the issue of vaccine safety and effectiveness must first be addressed. 

In the absence of a vaccine in the near to medium future it is sensible to take steps to increase immunity and we have included an update on this in the current enewsletter.

The Hong Kong Centre for Health Protection has identified the following risk groups for complications:

·        Pregnant women

·        Children up to 5 years old

·        Adults over 64 years old

·        People with chronic illness

·        People with weakened immunity

·        People who are obese

We recommend that patients in these categories seek medical advice if they develop flu symptoms. Although the majority of individuals will make a full recovery naturally, your doctor will discuss the potential benefit of anti viral drugs for individuals in the above groups.

Worldwide it has also been noticed that the obese have a higher chance of complications. Obesity is defined as a BMI of 30 or more. BMI is calculated as Weight (kg) / Height (m) /Height. If you have a BMI above or near 29 it would be a good time to lose weight.