Sugere estudo
As células imunológicas continuam se organizando para combater o coronavírus meses após a inoculação, relataram os cientistas
Apoorva Mandavilli, The New York Times
As vacinas feitas pela Pfizer-BioNTech e Moderna desencadeiam uma reação imunológica persistente no corpo e podem proteger contra o coronavírus por anos, relataram cientistas na segunda-feira.
As descobertas aumentam as evidências de que a maioria das pessoas imunizadas com as vacinas de RNA mensageiro talvez não precisem de reforços, desde que o vírus e suas variantes não evoluam muito além de suas formas atuais – o que não é garantido. As pessoas que se recuperaram da covid-19 antes de serem vacinadas talvez não precisem de reforços, mesmo que o vírus passe por uma transformação significativa.
“É um bom sinal de como nossa imunidade com esta vacina é durável”, disse Ali Ellebedy, imunologista da Universidade de Washington em St. Louis que liderou o estudo, o qual foi publicado na revista Nature.
O estudo não considerou a vacina contra o coronavírus feita pela Johnson & Johnson, mas Ellebedy disse esperar que a resposta imunológica seja menos durável do que a produzida pelas vacinas de RNA mensageiro.
No mês passado, Ellebedy e seus colegas relataram que, em pessoas que sobreviveram à covid-19, as células imunológicas que reconhecem o vírus permanecem quiescentes na medula óssea por pelo menos oito meses após a infecção. Um estudo realizado por outra equipe indicou que as chamadas células de memória, ou células B, continuam amadurecendo e se fortalecendo por pelo menos um ano após a infecção.
Com base nessas descobertas, os pesquisadores sugeriram que a imunidade pode durar anos, possivelmente a vida inteira, em pessoas que foram infectadas com o coronavírus e posteriormente vacinadas. Mas não estava claro se a vacinação por si só poderia ter um efeito similar de longa duração.
A equipe de Ellebedy tentou resolver essa questão olhando para a origem das células de memória: os nódulos linfáticos, onde as células imunológicas são treinadas para reconhecer e combater o vírus.
Após uma infecção ou vacinação, forma-se nos nódulos linfáticos uma estrutura especializada chamada centro germinativo. Essa estrutura é uma espécie de escola de elite para células B – um campo de treinamento onde elas ficam cada vez mais sofisticadas e aprendem a reconhecer um conjunto diversificado de sequências genéticas virais.
Quanto mais amplo for o alcance e quanto mais tempo essas células tiverem de treino, maior será a probabilidade de serem capazes de impedir as variantes do vírus que possam surgir.
“Todo mundo sempre se concentra na evolução do vírus – isso mostra que as células B estão fazendo a mesma coisa”, disse Marion Pepper, imunologista da Universidade de Washington em Seattle. “E isso vai proteger contra a evolução contínua do vírus, o que é realmente animador”.
Após a infecção pelo coronavírus, o centro germinativo se forma nos pulmões. Mas, após a vacinação, a educação das células ocorre nos gânglios linfáticos das axilas, ao alcance dos pesquisadores.
Ellebedy e seus colegas recrutaram 41 pessoas – entre elas oito com histórico de infecção pelo vírus – que foram imunizadas com duas doses da vacina Pfizer-BioNTech. A equipe extraiu amostras dos gânglios linfáticos de 14 dessas pessoas três, quatro, cinco, sete e quinze semanas após a primeira dose.
Esse trabalho meticuloso faz com que este seja um “estudo heroico”, disse Akiko Iwasaki, imunologista de Yale. “É muito difícil fazer esse tipo de análise cuidadosa em humanos ao longo do tempo”.
A equipe de Ellebedy descobriu que, quinze semanas após a primeira dose da vacina, o centro germinativo ainda estava altamente ativo em todos os 14 participantes e que o número de células de memória que reconheceram o coronavírus não tinha diminuído.
“O fato de as reações terem continuado por quase quatro meses após a vacinação é um sinal muito, muito bom”, disse Ellebedy. Os centros germinativos geralmente atingem o pico uma a duas semanas após a imunização e, em seguida, diminuem.
“Normalmente, em quatro a seis semanas, não sobra muita coisa”, disse Deepta Bhattacharya, imunologista da Universidade do Arizona. Mas os centros germinativos estimulados pelas vacinas de RNA mensageiro “ainda estão ativos meses depois e não diminuem muito na maioria das pessoas”.
Bhattacharya observou que a maior parte do que os cientistas sabem sobre a persistência dos centros germinativos se baseia em pesquisas com animais. O novo estudo é o primeiro a mostrar o que acontece em humanos após a vacinação.
Os resultados sugerem que a grande maioria das pessoas vacinadas estará protegida a longo prazo – pelo menos contra as variantes existentes do coronavírus. Mas adultos mais velhos, pessoas com sistema imunológico fraco e aqueles que tomam medicamentos que suprimem a imunidade podem precisar de reforços; pessoas que sobreviveram à covid-19 e foram posteriormente imunizadas talvez nunca precisem de doses de reforço.
É difícil prever exatamente quanto tempo durará a proteção das vacinas de RNA mensageiro. Na ausência de variantes que driblem a imunidade, em teoria a imunidade pode durar a vida toda, dizem os especialistas. Mas o vírus está claramente evoluindo.
“Qualquer coisa que realmente exigisse um reforço viria de uma variante, não de uma redução da imunidade”, disse Bhattacharya. “Eu simplesmente não vejo isso acontecendo”. /Tradução de Renato Prelorentzou
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