Lately, the news in the USA was dominated by articles covering the measles outbreaks in the country. The outbreak infected nearly 1,000 in over 26 states since May 24. On the other side of the world, in Congo, the Ebola outbreak has infected nearly 2,000 people. The defeating information is that it killed over 1,100 of those infected.
R0 — The Basic Reproduction Number
R0 is a number that shows the potential of potential diseases to infect the people who don’t have it yet. It also calculates the transmissibility of that disease, and just how dangerous it is for the entire population. To make things more simple, the number calculates how dangerous one infected person is for the unvaccinated part of the community. It will estimate how many people could potentially get contaged only because they were in touch with an infected individual.
Scientists often use this number to calculate the size of an outbreak, the actual danger of the infection, and to develop tactics on how to fight it back. Basically, every infection in nearer history has its R0. When calculating this number, there are three most important factors one should take into consideration. Scientists need to determine how long do people remain infected (this can be a matter of days or months), how often do the infected individual come in touch with the healthy ones, and the probability of them passing on the disease on others.
These three factors are naturally affected by various circumstances. For example, it considers the age of the infected individuals, the local environment, the political climate in a country, and most importantly, public health quality. In case of an outbreak, the R0 can differ from one country to another, depending on these factors and circumstances.
R0 Differs From One Disease to Another
Naturally, the transmissibility of one infection can be completely different from another. For example, the R0 for measles is extremely high, from 12 to 18. On the other hand, the Ebola factor is quite low, from 1,5 to 2,5. This is a good thing when you think about the horrible symptoms Ebola carries. But keep in mind that R0 doesn’t measure the deadliness number, only the power of a specific infection.
For example, the highest transmissibility rate comes from chickenpox, a disease not quite deadly thanks to the vaccine. But chickenpox and measles are airborne viruses, which means that one can get infected by merely breathing the same air as the carrier.
On the other hand, diseases such as Ebola can only be transmitted through bodily fluids, which makes the infecting rate significantly lower. The perfect example of an exception can be HIV; a virus that has a high transmissibility rate but is transferred via blood and sperm.
Another situation where the R0 comes in handy is calculating the number of the population that needs to be vaccinated in order to avoid the epidemics. This is what medical experts call “the herd immunity.” When it comes to measles, the general agreement is that 92% to 95% of the population needs to be vaccinated for the population to reach immunity. Considering Ebola cannot be transferred as easily as measles, only 42% to 63% of the specific community should be vaccinated.
Unfortunately, current vaccines against Ebola are not a 100% guarantee that they will prevent you from getting infected. So a significant group of researchers claim that the Ebola threshold needs to be higher to avoid an outbreak.
Nevertheless, as useful as R0 can be, it is not overly helpful once the outbreak becomes a reality and not just a theory. This number won’t be able to tell you whether and how much a vaccine works in the midst of an epidemy. For those scenarios, one should use other numbers, such as Rt. Although many scientists today say that R0 has its fallacies, they do believe it saved us from a lot of outbreak over the course of years. If anything, it helped with vaccines against measles and chickenpox — a disease that killed millions before the cure came.