# What are the chances of coinciding with a positive this Christmas?

Know the probability of directly or indirectly coinciding with a person who will test positive in covid-19 in the next 14 days after attending various social events is possible thanks to a simple mathematical model.

Developed by Samuel Morillas, professor of Applied Mathematics at the Polytechnic University of Valencia (UPV), in collaboration with the professor of the IES Al-Andalus de Almuñécar (Granada) Antonio Fernández-Baillo, the model puts numbers on the risks involved in attending various events.

"The idea is to put in numbers what we all know, that we must avoid social gatherings and spend a lot of time with people without a mask," Morillas tells EFE, adding that as the number of events and the number of attendees increases "The risk of having been in contact with a person who will be positive for covid increases."

The spreadsheet can be downloaded at this address of the UPV. Following the developed mathematical model, a person may know the risk they are taking when planning to attend a series of social gatherings, a maximum of eight.

"People must be aware of the risks they take when attend gatherings in social gatherings and where the mask and there is no safety distance", indicates the mathematician.

## Contagions at social events

He gives as an example that a person who goes to three different social events, in which he has been with 8, 9 and 10 people, respectively, the probability that he had contact with a positive in covid-19 it is 15 percent.

"After three dinners, almost in all probability the 99 percent have agreed with someone who has, in turn, with a person who will be positive in covid and the latter has also done it, in a chain ", he indicates. In the case of attending a single meeting in which there are thirty people, the probability goes up up to 17 percent.

According to Morillas, "the base probability to calculate all this is the cumulative incidence of covid-19 that we predict will be in the next 14 days in whatever context"." This model aims to numerically illustrate how the chances of directly or indirectly coinciding with a person who is positive for covid-19 increase, "he indicates.

The professor adds that it is designed to raise awareness about the need to limit the number of events and attendees social activities in which we will participate in the coming days. Although the model can be used for any type of event, it is designed for those held in closed places and during which the attendees do not wear the mask for a long period of time, cases in which the risk of contagion by covid is tall.

Its creators indicate that in no case does the model intend to predict chances of contagion but simply reflect how the possibilities of the same increase as the number of events and / or attendees increase. In addition, the model makes a forecast of the future incidence of positive for covid-19 and for this, the factor used is the incidence in the previous 14 days, so that it assumes that the growth rate remains constant, although in practice it depends on numerous factors.

The possibility of an indirect contact with a positive future is calculated in covid, that is, to coincide with someone who in turn has directly or indirectly coincided with a positive future. "With the term indirectly - it indicates - it refers to that potential chain of coincidences that could be a chain of contagions".

Although to calculate this accurately you should know the previous events each person attends, which implies an amount of data difficult to manage, it is assumed that all attendees go to the same number of events and with the same number of attendees. It must be taken into account that these contact probabilities "do not reflect contagion probabilities at all, as this depends on many more factors that are not considered in this model." These results are only intended to illustrate how the chances of contagion increase as the number of events and / or attendees increases.