Tropical regions need to revise their influenza vaccine strategy

Studies suggest that current WHO recommendations based on the hemisphere in which a country is located are not suitable for regions near the tropics, where the dynamics of influenza virus circulation are different.

By Karina Toledo

Agência FAPESP – Recent studies have shown that countries with long latitudinal spans, such as Brazil, need to adopt two distinct policies for influenza vaccines: one that follows the calendar recommended for the Southern Hemisphere and another that follows the calendar for the Northern Hemisphere.

This warning came from Nancy Cox, Director of the Influenza Division at the Centers for Disease Control (CDC) in the United States, in a recent editorial in the Bulletin of the World Health Organization, published by the World Health Organization (WHO).

The WHO currently recommends that countries situated in the Southern Hemisphere inoculate their populations between April and May and that those in the Northern Hemisphere inoculate between September and December.

In both cases, at the time that the vaccine is formulated, a prediction is made about the influenza strains that will be circulating during the winter months. This prediction is based on data collected by an international network for epidemiological surveillance led by the WHO.

“In all countries with temperate climates, a clear seasonal peak in influenza activity occurs during the winter months. In countries with tropical or subtropical climates, however, influenza seasonality is more variable, and influenza activity can be observed year-round, although it often is more intense during the rainy seasons,” Cox said in the editorial.

The CDC scientist commented on findings from a study conducted between 2006 and 2011 in 10 Asian countries, all situated in the Northern Hemisphere. The study, published in the same issue of the Bulletin of the World Health Organization, was performed in Bangladesh, Cambodia, India, Indonesia, Laos, Malaysia, the Philippines, Singapore, Thailand and Vietnam.

The analysis revealed year-round circulation of influenza viruses in all of the studied countries. However, two distinct patterns emerged. In Indonesia, Malaysia and Singapore, countries close to the Equator, there was no obvious peak in influenza activity. In the other seven countries – more northern but still in the tropical region – there was a distinct peak in influenza activity between July and October, the summer monsoon season (and summer in the Northern Hemisphere).

“This peak contrasted with the winter peak that typifies influenza seasonality in the more temperate areas of the Northern Hemisphere. A similar relationship between latitude and the timing of peak influenza activity has been observed in Brazil,” Cox said, referring to data from the first article to address this subject, published by Brazilian scientists in 2009 in PLOS ONE.

According to Wladimir Alonso, one of the study’s authors and a research fellow at the Fogarty International Center of the National Institutes of Health (NIH) in the United States, even before 2009, many experts suspected that influenza seasonality was different in tropical regions. “We wrote the first quantitative article on the topic,” he said.

The Brazilian group collected samples from flu-inflected patients at the Adolfo Lutz Institute in São Paulo and at the Evandro Chagas Institute in the state of Pará between 1999 and 2007. The strains found in the samples were then compared with strains that constituted a portion of the vaccine formulation used in the Southern Hemisphere in the corresponding year.

“In addition to comparing the historical scenario – in other words, comparing the vaccines that had been administered with the strains that were in fact in circulation – we analyzed a hypothetical scenario and observed that the vaccine formulation recommended for the Northern Hemisphere would have better captured the strains that existed in the samples at the time that the vaccine was offered,” Alonso said.

According to the researcher, this initial study indicated a closer match with the Northern Hemisphere vaccine for the samples collected in both Pará and the southeastern region.

“However, more recent studies conducted in collaboration with Fiocruz (RJ) have revealed that the recommendation made for the Southern Hemisphere is more suitable to Brazil’s southern and southeastern regions. In the North and Northeast, with the exception of the state of Bahia, it would be better to follow the calendar and formulation recommended for the Northern Hemisphere because they are locations where there is no well-defined winter and where the influenza circulation dynamic obeys climate factors that are not yet fully understood,” Alonso said. He also participated in a study that determined a similar situation in China.

Influenza circulation

According to Adolfo Lutz Institute researcher Terezinha Maria de Paiva, co-author of the PLOS ONE article, mapping to better understand the dynamics of influenza activity is currently being performed in the central-western region of Brazil.

“We have already received samples from the states of Mato Grosso, Mato Grosso do Sul, Goiás, Piauí, Tocantins and Rondônia. Currently, the data appear to be compatible with what has been observed in the South and Southeast,” Paiva said.

According to the assessment of Cox, Alonso and de Paiva, there is already enough evidence to change the vaccine strategy in several countries situated in tropical and subtropical regions, including Brazil.

“I think that it is really important that people who understand these data are able to make sensible policy decisions for their countries. In the case of India, for example, the vaccination period has already been changed. However, there are still some countries, and particularly the poorest countries in Asia, that have not yet adapted any vaccination strategy, despite the huge burden that flu represents in developing societies,” Cox told Agência FAPESP.