Pfizer and Coronavac prevent omicron hospitalization even 6 months after 2nd dose, says study

A survey carried out in Hong Kong with more than 164,000 participants pointed out that both the Pfizer vaccine and the Coronavac maintain high protection against hospitalization caused by omicron six months after application.

Protection to avoid deaths in the same period also remained high, although it showed a drop in the age group above 50 years. Protection against hospitalization and to avoid deaths in the population over 80 years old suffered a greater reduction in both types of vaccines.

The study was conducted by researchers from the Faculty of Medicine and the Center for Research in Health Policy and Health Systems, both at the Chinese University of Hong Kong, and the Department of Epidemiology at the Harvard TH Chan School of Public Health (Boston). The results were published in the scientific journal Jama Network Open.

To assess the so-called effectiveness (effectiveness in real life) of vaccines, scientists analyzed data from 32,832 cases of hospitalization or death caused by omicron from January 1 to June 5, 2022 and compared with 131,328 who did not have the disease (working like this as a control).

In the first six months after vaccination, the effectiveness of the primary vaccination schedule with two doses of Coronavac or Pfizer maintained similar levels of protection against hospitalization (86.4% for Coronavac and 92.9% for Pfizer) and death (74% and 77.4%, respectively) after the second dose.

After six months, a period in which there is a natural drop in the antibodies produced by the vaccine in both cases, the loss of immunity was greater in older individuals, over 80 years old (61.4% for protection against hospitalization due to Coronavac and 52.7% by Pfizer).

Finally, the study also evaluated the application of a booster dose and found similar effectiveness using only Coronavac or only Pfizer, but the heterologous booster (that is, when there is a mixture of immunizers) was better to protect against hospitalization and death (85 .1% and 87.3%, respectively).

The researchers’ conclusion is that a booster dose offers high protection against the development of severe Covid caused by omicron over time, as already observed in other studies.

The recommendation, therefore, is that although protection against hospitalization and death remains high in individuals under 50 years of age, those older may benefit from a third dose to prevent omicron and its underlining.

Another interesting fact was that the risk of hospitalization or death in unvaccinated individuals was twice as high compared to those vaccinated, which reinforces the importance of vaccination against Covid.

The research has some limitations, such as the fact that at the time of omicron’s arrival in Hong Kong, protective measures against the coronavirus were no longer in place, making it difficult to track cases and diagnose people who are asymptomatic or with mild symptoms. Another problem was the overcrowding of health care units, which may have caused a distortion in the data of people hospitalized due to Covid.

The authors reinforce, however, that few studies have been carried out with CoronaVac in the context of omicron and, in general, the reinforcement researches considered a scenario of monitoring patients with a shorter interval between doses.

At the beginning of omicron circulation, several researches pointed to a reduction in the neutralization capacity of vaccines against this strain. These studies were done in the laboratory using sera from vaccinated individuals, but data on real-life efficacy were not available.

“Compared to the level of antibodies after two doses of Pfizer [que decaiu 4,7 vezes em 5 a 6 meses]individuals vaccinated with Coronavac did have a greater reduction in the rate of neutralizing antibodies [de 6 a 10 vezes menor em seis meses], potentially indicating a lower efficacy. However, vaccinated individuals maintained cellular immunity capable of protecting against the progression of severe Covid even after the natural decay of antibodies,” the authors wrote in the paper.