This week’s coronavirus column

Published Thursday 26 August 2021 at 15:41

Our Director of Public Health and Wellbeing, Professor Dominic Harrison, has written another column for the Lancashire Telegraph.

Read it here:

Herd immunity

On Monday 23rd August, Blackburn with Darwen had the lowest Covid case rate of any upper tier Local Authority in England. The Boroughs rate was at 187 per 100,000. The highest UK Covid case rate was in Hull at 637, and the England average was 310.

The Borough is still at the top of the national UK league table for its ‘cumulative case rate’. This means that to date, the Borough has had the most people in the UK, as a percentage of its overall population, who have had a confirmed diagnosis of COVID 19 since the pandemic began. Burnley, Hyndburn and Pendle are also amongst the UK Local Authority areas with the highest cumulative case rates.

What might this mean for herd immunity?

Herd immunity is a bit of a slippery concept. Essentially as the medical dictionary defines it, the ‘herd immunity threshold’ is the proportion of a population that need to be immune in order for an infectious disease to become stable in that community. For the Delta variant this may be between 90-93% of the population. If this point is reached— either by vaccinated immunity or immunity from previous exposure—then at that point, each case leads to only one other new case and the infection will become stable within the population. It will not increase, and the famous R number will be 1 (R=1). If immunity in the population goes above the herd immunity threshold then R will fall below one and, in theory at least, the disease eventually dies out as each single infected person infects less than one other person until it disappears.

In practice however, Covid will not disappear. Even if an area has very high immunity, that immunity is unlikely to be spread equally across the whole population. Some areas may be well above the herd immunity threshold but some areas below it – so the disease can continue to spread ‘endemically’ – in smaller communities or neighbourhoods for years as is the case  with Tuberculosis.

Given Blackburn with Darwens ‘near to national average’ vaccination rate, and its status as the highest UK confirmed Covid case rate area, the borough has a high likelihood of being one of the first Local Authority areas in the UK to achieve population herd immunity. We are not yet at the population herd immunity threshold- but we have amongst the best chances of any Local Authority in the UK of getting there.

As of Tuesday this week, the Borough had 26,426 confirmed covid cases but this number will not include everyone who was a case from the start of the pandemic. From March 2020 until May 28th 2020, Covid testing was only available if you were a case in a care home or hospital. It will not include all of those who were infected but asymptomatic, who did not know they were a ‘case’, and did not get tested. It will not include everyone who had some symptoms but chose not to get tested. The real case rate in the Borough may well be at least 50,000 – one third of our population of 150,000.

If everyone eligible in every neighbourhood gets vaccinated and if the government agrees to extend vaccination to all of our residents over 12 years old, herd immunity could be a real possibility-locally at least. We will still have cases, and the virus will be circulating for years especially in winter – but we will have exited both the pandemic and epidemic phase and deaths will be minimal. To get there, the next four months need to be all about vaccination, vaccination, vaccination……….!


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