¿Y si la inmunidad de rebaño está más cerca de lo que creían los científicos?

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La policía local patrulla y controla que las personas utilicen tapabocas en la Plaza de España, en Roma, el 17 de agosto de 2020
La policía local patrulla y controla que las personas utilicen tapabocas en la Plaza de España, en Roma, el 17 de agosto de 2020 Fuente: AFP

Sabemos desde el principio cómo habría de llegar el final. En algún momento, el coronavirus  ya no podrá encontrar huéspedes donde alojarse para sobrevivir, y se extinguirá allí donde resurja fugazmente.

Para alcanzar la así llamada inmunidad de rebano -ese punto en el que el virus ya no se propaga porque no encuentra suficientes humanos vulnerables al contagio-, los científicos han sugerido que tal vez haga falta que un 70% de la población sea inmune, ya sea por vacunación o por haberse curado de la enfermedad.

Ahora, algunos investigadores están barajando una posibilidad más esperanzadora. Más de una docena de científicos entrevistados por el diario The New York Times dicen que ese umbral probablemente sea mucho más bajo: apenas un 50%, o incluso menos. De ser cierto, podríamos dejar atrás el coronavirus más rápido de lo que pensábamos.

Las nuevas estimaciones son resultado de complejos modelos de proyección estadística de la pandemia, y todos esos modelos han seguido abordajes divergentes, arrojando estimaciones que no son coincidentes. No hay certeza de que alguna comunidad del mundo tenga suficientes habitantes que ya sean inmunes como para repeler una segunda oleada de contagios.

Pero en algunas partes de Nueva York, Londres, y Bombay, por ejemplo, no es inconcebible que ya existe una significativa inmunidad contra el virus, aseguran los científicos.

“Estoy preparado para creer que hay bolsones poblacionales en Nueva York y en Londres donde la inmunidad es significativa”, dice Bill Hanage, epidemiólogo de la Escuela T.H. Chan de Salud Pública de la Universidad de Harvard. “Lo que suceda en el invierno que está por comenzar en el hemisferio norte será reflejo de esa inmunidad”.

Casi el 25% de la población de Nueva Delhi tuvo coronavirus, según un estudio del National Center for Disease Control publicado en julio que pone en duda las cifras oficiales de contagios en la India
Casi el 25% de la población de Nueva Delhi tuvo coronavirus, según un estudio del National Center for Disease Control publicado en julio que pone en duda las cifras oficiales de contagios en la India Fuente: AFP

“Pero el interrogante sobre lo que eso significa para el total de la población, sin embargo, es más peliagudo”, agrega Hanage.

La inmunidad de manada se calcula a partir del R0, el así llamado numero básico de reproducción del virus , que indica a cuántas personas sanas contagia cada infectado.

Los primeros cálculos del umbral de inmunidad de manada se hicieron asumiendo que cada miembro de la comunidad era igualmente susceptible al virus, y mezclándolos aleatoriamente.

“Eso no ocurre en la vida real”, dice el doctor Saad Omer, director del Instituto de Salud Global de la Universidad de Yale. “La inmunidad de manada puede variar de un grupo a otro, de una subpoblación a otra”, y hasta de un código postal a otro, señala Omer.

Por ejemplo, un barrio de adultos mayores puede tener poco contacto con gente de afuera, pero sucumbir al virus cuando se topa con él, mientras que los adolescentes pueden pasarles el virus a decenas de contactos sin caer enfermos. El virus avanza lentamente en las zonas rurales y suburbios residenciales, donde la gente vive menos amontonada, pero arrasa en los conglomerados urbanos y en los hogares hacinados.

Cuando se incorporan al cálculo esas variaciones de densidad y composición demográficas del mundo real, la estimación del número necesario para alcanzar la inmunidad de manada disminuye. Algunos investigadores incluso sugieren que esa cifra podría estar entre el 10% y el 20%, pero son minoría.

Asumiendo que el virus detecta a los más susceptibles y los más expuestos en la primera ola, la inmunidad que sigue a una ola de contagios queda distribuida más eficientemente que con una campaña de vacunación que quiere proteger a todo el mundo, dice Tom Britton, matemático de la Universidad de Estocolmo.

El modelo de Britton ubica la inmunidad de manada en torno al 43%, o sea que el virus no sobrevive en una comunidad donde la cantidad de recuperados de la enfermedad supera ese porcentaje.

Sin embargo, eso implica que muchas personas de esa comunidad se enfermaron o murieron, un precio a pagar muy alto para conseguir la inmunidad de manada. Y hay expertos, como Hanage, que advierten que ni siquiera una comunidad que haya alcanzado esa inmunidad puede dejarse estar, porque el virus puede reaparecer en fogonazos, acá o allá, por más que su propagación general se haya apagado.

Además, tampoco se sabe cuánto dura la inmunidad de los recuperados de la enfermedad.

“No estamos ni remotamente cerca de volver a la normalidad en nuestras vidas diarias”, dice Virginia Pitzer, epidemióloga matemática de la Escuela de Salud Pública de Yale. “Pensar que podemos dejar de cuidarnos y volver a la vida de antes sin que haya consecuencias y aumenten los casos, en un error.”

Las personas se alinean para hacerse una prueba de Covid-19 en el vecindario de Sunset Park, que ha experimentado un aumento en los casos de coronavirus en los últimos días, el 13 de agosto de 2020 en la ciudad de Nueva York
Las personas se alinean para hacerse una prueba de Covid-19 en el vecindario de Sunset Park, que ha experimentado un aumento en los casos de coronavirus en los últimos días, el 13 de agosto de 2020 en la ciudad de Nueva York Fuente: AFP

Una segunda ola  también golpearía a grupos o comunidades que se salvaron de la primera, y desatar una tragedia, dice Pitzer. En Nueva York, por ejemplo, la inmunidad está toda parcelada: el 68% de los pacientes que pasan por la clínica Corona, en el barrio de Queens, ya tienen anticuerpos, pero en Brooklyn, en una clínica de Cobble Hills, la cifra es de apenas el 13%.

Pero otro grupo, liderado por la matemática Gabriela Gomes, de la Universidad de Strathclyde, Gran bretaña, también incluyo esas variables en su modelo y descubrió que Bélgica, Inglaterra, Portugal y España tenían umbrales de inmunidad de manada de entre el 10% y el 20%.

“Al menos en esos países, no encontramos signos de que el umbral sea más alto que ese”, dice Gomes. “Creo que es un buen horizonte saber que nos quedan pocos meses de pandemia.”

Otros expertos advierten que esos modelos son fallidos y que subestiman y simplifican las condiciones reales. “El que diga que esto pasó, está jugando con fuego”, dice Jeffrey Shaman, epidemiólogo de la Universidad de Columbia.

The New York Times

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