The mirage of herd immunity
Weeks ago, I’ve already been writing about the untenability of the pursuit of herd immunity here in the Philippines.
This was primarily because of the lower efficacy of the vaccines that we’re hinging our response on: Sinovac, which represents 70% of the total vaccine supply of the Philippines, has a 50.7% efficacy based on Phase III trial results. Although real-world data have been more positive, especially in populations where more than 90% have been immunized, the data in Chile and Uruguay have revealed a real-world effectiveness against symptomatic infection at 65 and 57 percent, respectively. The studies, however, have provided that Sinovac is protective against severe infection and hospitalization, which is good. Is this enough to attain herd immunity, however, or to get rid of masks and other minimum health protocols?
My answer is a definitive no.
In a study published just a week ago, David Hodgson, with two of the more notable epidemiologists in the UK (Mark Jit and Adam Kucharski), have written a study regarding the potential for vaccination-induced herd immunity against the Alpha variant of SARS-CoV-2, and the results, as expected, are not promising.
According to the study, the feasibility of attaining vaccination-induced herd immunity depends on three factors: vaccine effectiveness in reducing transmission; the transmissibility of the target pathogen; and, vaccine coverage that is achievable in a population. Most importantly, “if vaccine effectiveness is below the threshold for herd immunity, even vaccination of the entire population would, on its own, be insufficient to ensure control.”
A certain “expert” has posted regarding Sinovac’s effectiveness in two studies, naming the Indonesian health care worker study and the Serrana study in Brazil. However, he failed to mention that almost everyone in that study was immunized. More than 90% of the individuals in the Indonesian study were immunized, and more than 95% of the adults in the Serrana study were inoculated. For a real-world study, such levels of immunization are unrealistic.
For Sinovac, a more realistic estimate of its effectiveness given the data in Chile and Uruguay would be 60%, and according to the Hodgson study, it’s not enough: “Our observations suggest that if highly transmissible variants become dominant in areas with low seroprevalence, and/or in populations with a high proportion of children, control of infection by vaccination, in the absence of non-pharmaceutical interventions, may only be achievable with a vaccine effectiveness against infectiousness of ≥ 80%.”
One major concern in the Philippines is that it has 33% of its population as children < 15 years of age. Because of the greater nature of transmissibility of the Alpha variant, higher levels of immunity among children would also have to be obtained, either through vaccination or infection. Basically, in the Philippines, with Sinovac as our primary vaccine, herd immunity is basically impossible. The study has admitted itself to be optimistic because it has not taken antibody waning and immunosenescence into account, like what the study by Khoury et. al have pointed out.
This study has led me to a few conclusions:
1) Herd immunity is unlikely with our current vaccines, and it is imperative that vaccines with better effectiveness (mRNA vaccines, Novavax) be obtained sooner rather than later.
2) Prioritizing the most vulnerable groups, given the limited amount of vaccines, is a good approach of the Philippine government, given the limited vaccine amounts. I think providing the economic frontliners with mRNA vaccines is one alternative, since they will be the ones that will travel around the most. Medical frontliners, because they are more knowledgeable with respect to disease transmission, and are more aware, may be provided with the viral-vector vaccines (not Sputnik). Preferably, however, they will also be provided with the mRNA vaccines.
3) A study in Hong Kong by Dr. Ivan Hung regarding the efficacy of Sinovac and Pfizer combination has been started. This will hopefully provide good results, so that the economic frontliners may be provided with a combination, while the most vulnerable and the medical frontliners can be provided with a Pfizer/Pfizer combination.
4) “Population protection,” as funny as it sounds, is likely the most realistic goal for a country with a significant amount of vaccine hesitancy as the Philippines. This is why I’m hoping for better vaccines, because vaccinating the 50% with the best vaccines is likely going to curb transmission and infectiousness even against variants, which only the mRNA vaccines have shown to be effective against, instead of using vaccines that do protect against severe disease and hospitalization but do not stall infectiousness or transmission.
