The advance of measles in Mexico has set off alarms. The presence of cases in all the states of the country that confirm the widespread transmission of the virus has accelerated vaccination campaigns and the imposition of prevention measures in entities such as Jalisco, where the majority of cases are concentrated. While the possibility of losing the status of a country free of the disease before the Pan American Health Organization (PAHO) looms on the horizon, questions about the nature of the virus, the risk of contagion and vaccination schedules are skyrocketing on search engines and social networks.

How measles is transmitted

Like any other respiratory virus, measles is spread through the air, by inhaling microscopic droplets expelled by a sick person when they speak, cough or sneeze, which remain suspended in the air and contain a sufficient load of the virus to cause infection. Epidemiologists use an indicator called the basic reproductive number—represented as R0—to see how easily an infectious disease spreads. “The basic reproductive number defines how many infections a person who is sick can cause in a population that does not have protection,” Mauricio Rodríguez Álvarez, spokesperson for the UNAM epidemiological risk program, explains to this newspaper. For measles, the R0 is calculated between 16 and 18, “this means that a person with measles could infect up to 18 people who were not protected. With this, we say that it is the most contagious virus there is,” he assures, emphasizing that the calculation is always based on an optimal transmission scenario, “in which no measure is implemented to stop it.”

To measure how contagious measles is, Rodríguez gives the example of covid-19 in one of the moments of greatest transmissibility of the pandemic, the summer of 2021 in the middle of the wave caused by the delta variant: “The virus mutated and had characteristics that allowed it to be more contagious, and it is estimated that there one person could infect four.” In the case of seasonal influenza, he explains, the times of greatest activity during epidemics yield an R0 between 1.5 and 2.5.

Do prevention measures other than vaccination work?

Rodríguez is categorical on this point: “It should not be an action that excludes the other, but rather adds: not by saying ‘let’s vaccinate’, then let’s neglect the measures to avoid infections, and not by saying ‘let’s put on face masks’, let’s neglect vaccination. We have to move forward on these two fronts,” he explains. In Jalisco, the State with the most active cases in the country, the state government established the use of masks in schools in the municipalities that concentrate the most cases, the same as the State of Mexico, the most populated in the country.

Although both fronts complement each other, when a virus begins to circulate it is necessary to reinforce both immunization and the measures to stop infections, the same ones that became popular during the new coronavirus pandemic. “When you do not have transmission of the virus, everything is vaccination, but when you have it, you must vaccinate, you have to be extremely vigilant and avoid contagion in the community by applying general measures: face masks for the sick, isolating yourself in case of illness, hygiene of hands and surfaces, of the objects and articles used by the sick and ventilation,” he says.

Why those over 49 years of age are more protected

Unlike the usual scenario with respiratory diseases such as Covid-19 and seasonal influenza, where the priority in vaccination campaigns is placed on older adults, the case of measles is particular: the Government has established that people over 50 years of age do not need to be vaccinated, since the high circulation of the virus in the last half century has given them lasting and effective immunity. “Those over 50 years of age have protection due to their life history, because they lived their childhood at an epidemiological time in which there were many cases of measles. During the decades of the fifties, sixties, seventies and eighties the virus circulated widely and all those who at that time were boys and girls became infected, faced the virus in some way, knew it and developed immunity; they survived and today have lasting protection,” says Rodríguez.

In the 1970s, the introduction of the measles vaccine caused drops in the incidence of the virus, at the same time it extended the coverage of vaccinated people, a scenario in which the protection of the population stopped being a consequence of having resolved the natural disease and began to be a consequence of vaccination, explains Rodríguez. “The immunity conferred by having had the natural disease is better than that of the vaccine, because the body responds differently to virulent viruses than to the attenuated viruses of vaccines.” The infectious diseases expert explains that all studies that evaluate the level of antibodies in people at a population level indicate that the age group over 50 years of age presents “sufficient protective levels, that they are not at risk, as the youngest ones are.”

Who should be vaccinated

Based on the evidence analyzed over decades, vaccination guidelines have changed over time and with it, the schemes to achieve the best protection against measles. In Mexico, the most recent studies confirm the immunity acquired by those over 50 years of age and the need to vaccinate the population outside this age group that did not receive a complete schedule, that is, two doses of the triple viral vaccine, which contains the attenuated measles, mumps and rubella viruses, explains Rodríguez. People who were vaccinated in the last six years are also protected and do not need to receive another booster dose.

“What we are now seeing thanks to the work of the National Health and Nutrition Survey is a population between 18 and 49 years old that has insufficient levels of protection. This is how it was known last year that there was a population group over 50, which did have sufficient protection, and a group of people under 49 who had insufficient protection. We also know that the group between 20 and 39 years old is the one with the lowest protection,” he says, while calling for vaccination as a mechanism. effective and safe to stop infections. “They are vaccines that have been used routinely and in emergency situations for more than 50 years in a safe and effective way,” he concludes.