Facts and figures on ‘super spreaders’ and vaccines

Facts and figures on ‘super spreaders’ and vaccines
Medical Matters


As we move to into the next phase of the ‘unwinding’ of the coronavirus lockdown it is apparent that restrictions of some sort will remain with us for some time. Covid-19 has been the focus of many scientific articles and studies but while there are still many unknowns, we have learned much that perhaps may have changed our approach to tackling the virus.

Most of the discussion around the spread of SARS-CoV-2 has centred on the average number of new infections passed on by each patient or the reproduction number (R0). Without social distancing, this number is about 3.0 but is now estimated to be about 0.5 in Ireland. However, in real life some people infect few or no people while others pass it on to many. As a case in point, a patient in France had Covid-19 as early as December but this didn’t lead to more widespread infection.

This has drawn scientists to look at a different measure called the ‘dispersion factor’ (k) which reflects how much a disease clusters. The lower the value the more the transmission comes from a smaller number of people.  We know that the spread of this coronavirus is in many cases from fewer people than the flu. In fact, it is now estimated that up to 80% of cases may result from spread from only 10% of people meaning that restrictions like widespread lockdown is a very blunt tool.  Indeed, the notion of one person spreading the disease to potentially up to 100’s of others has given to rise to the term ‘super spreader’. In fact, this appears to account for significant outbreaks in choir groups, meat factories, night clubs and food markets.

But why might some people become super spreaders?  Many factors are likely to come into play. Firstly, some people will shed more virus than others and of course this also depends on what stage the infection is at and whether they’re coughing. Additionally, the amount of virus projected into the air is probably greater when shouting or singing. In fact, a study of healthy people in 2019 suggested that even those who speak louder are more likely to project viruses further.

What’s also a big concern is that as well as droplet spread, the virus can probably be picked up from aerosols suspended in the air. This is particularly problematic when indoors or in enclosed spaces. Indeed, a study in Japan found that the risk of infection was 19 times greater when comparing indoors versus outdoors.

But what about the two-metre distancing rule? It probably originates from a study examining cough droplets in the 1930’s. The WHO has cited that 70% of droplets will travel one metre and that two metre keeps you safe 99% of the time. Of course, the virus can spread further but when outside the risk is low. Some countries like Germany, Poland and the Netherlands have adopted 1.5 metre rule and yet others such as Sweden, Norway and Austria only one metre.

On a positive note, a recent study suggested that infection with the common cold may provide some immunity against Covid-19. Scientists found a that a significant proportion of people (who had blood tests taken between 2015-2018) and hence did not have Covid-19 had specific immune cells (T helper cells) that recognised the virus. About 20% of the time, the common cold is caused by a coronavirus virus that has similarities to SARS Cov-2 which may help explain the findings.

News on the vaccine front has been mixed. There are more than 100 pre-clinical studies exploring vaccine development. However, only a handful have progressed into real clinical trials with patients. Ultimately, the goal is getting the body to produce antibodies that will latch on to the virus and stop it from entering cells.

The early results of two trials that used a vector model are encouraging. Essentially, components of the current coronavirus are put into a weakened cold virus which is then inoculated into an individual.  In a Chinese phase one trial published in the Lancet where 108 people were vaccinated, they observed neutralising antibodies at day 14, peaking at day 28 with no major adverse reactions.

In Oxford, vaccination in a similar way in Chimpanzees resulted in antibodies within 28 days and prevention of lung damage when exposed to the virus. However, it didn’t completely stop virus replication. This vaccine is now in clinical trials though ironically as the level of the virus in the community in the UK is now low, the number of people likely to get infected may be insufficient to gauge a clear comparison between vaccinated and non-vaccinated groups.

Finally, as regards, treatments, there is now good evidence for the anti-viral Remdesivir. It was shown to shorten time to recovery by four days in a trial involving 1063 patients supporting its use for those in hospital on oxygen therapy.

Unfortunately, the largest study of hydroxychloroquine with or without azithromycin that involved 9237 treated patients showed an increased risk of death by about 10-15%. A trial is also underway into the drug Ivermectin which was promising in pre-clinical studies.

Ultimately, we will need to wait for a vaccine and that is not likely until next year!

Dr Kevin McCarroll is a Consultant Physician in Geriatric Medicine, St James’s Hospital, Dublin.