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The dangers of ‘natural’ herd immunity


People wearing face masks amid concerns over the spread of the coronavirus walk in Shinagawa station in Tokyo. AFP

People wearing face masks amid concerns over the spread of the coronavirus walk in Shinagawa station in Tokyo. AFP

As cases of Covid-19 pass the three million mark, the race for an effective and safe vaccine is on. There are now over 100 vaccine candidates, and at least one vaccine is beginning Phase 2 clinical trials. Even the most optimistic of projections, however, put the earliest availability of a vaccine for mass use to the start of 2021.

Why does it take so long to develop a vaccine? And why can’t we use vaccines that have already been developed for related diseases like SARS and MERS? Is “natural” herd immunity a viable option? Why not just get it all over with? These are all very good questions with complex answers and a great deal of uncertainty.

  1. How long does it take to develop a vaccine?

Vaccines typically take five to 10 years to develop. Developing a vaccine starts with a preclinical phase. This is when scientists identify a strategy for safely inducing immunity to a disease-causing organism. The immune system is made up of many arms, but the two main components that are important in vaccine development are the humoral arm and the cell-mediated arm. The humoral arm produces antibodies. The cell-mediated arm identifies viruses and other pathogens that are hiding inside our cells and eliminates them. The strategy for inducing humoral and cell-mediated immunity varies per pathogen, depending on the way that a virus, bacteria, or other organism causes disease.

For instance, immunity to measles is best induced with a weakened live virus, because a dead virus will not stimulate enough of an immune response to be protective. Vaccines for hepatitis B use a recombinant, or manufactured piece of the virus called HBsAg. HBsAg works because the antibody produced against it sticks to the surface of the hepatitis B virus and prevents it from infecting cells.

Once scientists find a good target for the vaccine, they have to go through three clinical phases prior to clinical use. Phase 1 tests the vaccine for safety in humans. Phase 2 looks at how well the vaccine generates antibodies and at which dose. Phase 3 proves that it is effective in the target population, and looks at what side effects may be significant.

We now have one vaccine against Covid-19 in Phase 2 and a handful in Phase 1. The rest are in the preclinical stage.

  1. Why not just use vaccines developed against SARS and MERS?

While SARS-CoV-2, the virus that causes Covid-19 is related to MERS-CoV and SARS-CoV (CoV stands for coronavirus), it is not exactly the same. Vaccines to SARS and MERS never progressed to a Phase 2 or 3 trial because those outbreaks ended naturally. There would not have been any way to do a Phase 3 trial because there were not enough cases to test it against by the time the vaccine was ready.

SARS-CoV and MERS-CoV are similar enough to SARS-CoV-2 such that vaccines for the older viruses have been used as the backbone for a SARS-CoV-2 vaccine, with some structural tweaks. Repurposing the old vaccines accelerated some of the preclinical work on the SARS-CoV-2 vaccines. There are already some vaccine candidates based on old MERS vaccines undergoing Phase 1 testing.

  1. If it takes so long to develop a vaccine, why don’t we just let people get sick and become immune? Why not pick the ones who are least likely to have complications and let them get the disease, so they become immune and protect the vulnerable?

This is the premise of proponents of natural “herd” immunity. The idea is, if around 70 percent of the population gets sick and develops antibodies, the epidemic will be over. There are multiple problems with this approach.

First, there is no guarantee of long-lasting immunity to Covid-19. Some preliminary reports have shown that there are people with very little or no antibody response to Covid-19 despite full recovery. If immunity does develop, it is unclear that it will last more than a few months.

Second, it is unethical to expose a population to an unknown disease without giving them the means to protect themselves. While most young people who get Covid-19 do recover, some have died and others have become critically ill.

Third, many young people (especially in the Philippines) live in multi-generational households. They can bring the virus home to vulnerable senior family members with disastrous consequences.

Fourth, the more people the virus infects, the higher the chances it has to mutate into a serotype that can escape herd immunity. This will start off another wave of infections. This can also potentially escape the effect of vaccines that are in development.

Finally, since there is currently little immunity to the virus, allowing people to get infected will result in a surge of cases that will overwhelm healthcare systems. Many people who need to be in the hospital will just die at home or while waiting for care in the emergency room.

Currently, only sick people tend to get tested for Covid-19. There has been a suggestion that if people with very mild symptoms of Covid-19 were included in the numbers, then the true death rate would be less than that of influenza. This is patently false. Influenza typically kills 50,000 people a year in the US. Covid-19 deaths have already surpassed 50,000 in that country in one month. Covid-19 is undoubtedly more deadly and more complicated than the flu.

We know so little about this virus and it continues to surprise us. Until a vaccine is found, our best defense against it is prevention—wear a mask, practice physical distancing, wash your hands, and stay at home as much as possible.

There are many drugs undergoing clinical trials as we speak, and an effective medication may be found in as little as one to three months. This will further reduce deaths and will allow us to be more confident about slowly opening society. Until an effective vaccine is found, however, we need to stick to the new normal. This is the only way to prevent further damage from a virus that has already cost us dearly.

About Dr. Salvana:

Edsel Maurice T. Salvana, MD, DTM&H, FPCP, FIDSA is an internationally recognized infectious diseases specialist and molecular biologist at the University of the Philippines and the Philippine General Hospital. He is the director of the Institute of Molecular Biology and Biotechnology at the National Institutes of Health at UP Manila. He has spoken and written extensively on the Covid-19 outbreak, and serves on the Technical Advisory Group of the Inter-Agency Task Force for the Management of Emerging Infectious Diseases (IATF-EID).

Source: Manila Bulletin (

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